[virtcon] Don't update framebuffer with no display

This change reduces how much work the virtcon does when it doesn't have
ownership of the display. First, add checks for ownership to all checks
for the presence of a display (i.e. vc_gfx being non-null), so we treat
not having ownership the same as not having a framebuffer to render to.
Second, add ownership checks to all checks for vc->active, so we treat
not having ownership as all virtcons being inactive.

Test: Check that output during no display appears when focus is gained
Change-Id: Ib8ca77b0c9a104131bd37756878b30f7136c1b38

[vim-display][display-test] Support NV12 layers

The single fullscreen layer can now use an NV12 image.

Test: display-test

Change-Id: Ie85f91bb4b1ecd5e476012af14fc9d872cccfc46

[virtcon] Fix incorrect usage of display_id

Test: Boot to virtcon
Change-Id: I9c7c214cd5dcb4bf09c8cba41cbcee865f68a7a7

[virtcon] Try to show virtcon on all displays

This change makes the virtcon try to use all connected displays. One
display is picked as the primary display, and the image created for
that display is mirrored to all other connected displays.

Test: Boot virtcon with multiple displays; connect a new display while
the virtcon is running; unplug the primary display; unplug a secondary
display (all tests run on NUC and NUC with scaling disabled).
Change-Id: I80aeb940fa2276f6dd64649f20104c493eacfc0a

[display] Add display mode validation

Test: display-test on NUC and eve
Change-Id: Ic5f35bbd73b3d860d330cc94102009ab9ba8b45c

[syscalls] Remove read_status from zx_channel_call

This parameter no longer has a purpose. Also, remove ZX_ERR_CALL_FAILED.
This status code no longer has a purpose.

Test: No behavior change
Change-Id: I71945ea9d1d6d591a20476106df1e598dfaf97b1

[syscalls] zx_channel_call should always consume handles

Previously, whether zx_channel_call consumed handles depended on whether
it succeeded or failed.

ZX-2204 #comment

Test: utest/core/channel

Change-Id: Ic7b1149a85a1a892144479ee2f543fc7fedc6114

[virtcon] Add a flag to hide the virtcon on boot

This change adds the virtcon.hide-on-boot kernel cmdline argument that,
if present, hides the virtcon on boot until some user interaction
explicitly shows it. This is useful to stop the virtcon from flashing
on boot if there is some garnet+ UI.

Bug: ZX-2262
Test: Manual boot with/without the flag
Change-Id: I596a2c80fb27a79e96705cec0842bdd3e8921d4a

[virtcon] Use display layer API

Test: boot to virtcon
Change-Id: Ie61fb8d5175518130e40fe9484bbccbee48a685a

[display] Add support for layers to core driver

Change-Id: I20f632c2921e00157d2c2278ce8a2e023aa5d5a9

[display] Move the display fidl library into the fuchsia namespace

Change-Id: I92d1e27f8341291292e1969f62d87f457ae76ddc

[virtcon] Migrate to new display APIs

Change-Id: Ia204796d3e52a7ca934fc171f4a231c89110dc1f