[virtcon] Avoid reading from WC memory

Reading from memory with a cache policy other than CACHED is
potentially very slow. Since gfx_surface needs to read from its buffer
for scrolling, if the framebuffer vmo can't be mapped CACHED then use a
second gfx_surface for rendering and blit the result to the framebuffer
vmo.

This change relies on the display driver setting an appropriate cache
policy with zx_set_cache_policy and keeping its own mapping of the VMO
to prevent the virtcon from setting the cache policy.

Bug: ZX-2198
Test: Check virtcon scrolling speed on NUC with intel_i915 disabled
Change-Id: I8af71b72ed9fff720947e685dee286d9c0edaf26

[display] Remove ddk calls from on_displays_changed

This change replaces get_info with per-display argument structs to
on_displays_changed, and it moves the display mode validation to a
posted task. This allow simpler locking in the impl drivers, and it
enables the addition of output parameters into the argument structs.

This change also causes ownership messages to be sent before display
changed messages. Update various clients to not rely on a particular
order of messages.

Test: Boot to virtcon/hotplug on NUC/vim/eve. Run gfxtest, gfxlatency,
and display-test

Change-Id: Id561ed7587037353c1aeeab96511481d74026bdc

[display] Add display mode validation

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

[simple-display] Allow consecutive display clients

ZX-2205 #done

Test: Run gfxtest twice in qemu with virtcon.disable
Change-Id: I39de34c0678722808f35122fb74873a41ef534c9

[display] Add layers to ddk display protocol

Change-Id: I0e876353121ca8c58bcca4a5c0ada6ee61d0e259

[ddk][display] Add support for non-edid displays

Change-Id: Idabcda75709659363deb31cead1f2b923a8b6fcc

[display] Convert amd-kaveri and nv to simple drivers

Change-Id: Ic95616a8832c51bf46768ecf84e1049524ad6e28

[display][simple] Create a dumb display driver

Convert the bochs display driver to the new display controller protocol
in such a way that most code can be shared with other dumb display
drivers.

Change-Id: I65b40203bb6c871d25a0c4be9d3d71b436b65ce7