drm/crc: Only report a single overflow when a CRC fd is opened

This reduces the amount of spam when you debug a CRC reading
program.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Change bool overflow to was_overflow (Ville)]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180418125121.72081-1-maarten.lankhorst@linux.intel.com

drm: Add API for capturing frame CRCs

Adds files and directories to debugfs for controlling and reading frame
CRCs, per CRTC:

dri/0/crtc-0/crc
dri/0/crtc-0/crc/control
dri/0/crtc-0/crc/data

Drivers can implement the set_crc_source callback() in drm_crtc_funcs to
start and stop generating frame CRCs and can add entries to the output
by calling drm_crtc_add_crc_entry.

v2:
- Lots of good fixes suggested by Thierry.
- Added documentation.
- Changed the debugfs layout.
- Moved to allocate the entries circular queue once when frame
generation gets enabled for the first time.
v3:
- Use the control file just to select the source, and start and stop
capture when the data file is opened and closed, respectively.
- Make variable the number of CRC values per entry, per source.
- Allocate entries queue each time we start capturing as now there
isn't a fixed number of CRC values per entry.
- Store the frame counter in the data file as a 8-digit hex number.
- For sources that cannot provide useful frame numbers, place
XXXXXXXX in the frame field.

v4:
- Build only if CONFIG_DEBUG_FS is enabled.
- Use memdup_user_nul.
- Consolidate calculation of the size of an entry in a helper.
- Add 0x prefix to hex numbers in the data file.
- Remove unnecessary snprintf and strlen usage in read callback.

v5:
- Made the crcs array in drm_crtc_crc_entry fixed-size
- Lots of other smaller improvements suggested by Emil Velikov

v7:
- Move definition of drm_debugfs_crtc_crc_add to drm_internal.h

v8:
- Call debugfs_remove_recursive when we fail to create the minor
device

v9:
- Register the debugfs directory for a crtc from
drm_crtc_register_all()

v10:
- Don't let debugfs failures interrupt CRTC registration (Emil
Velikov)

v11:
- Remove extra brace that broke compilation. Sorry!

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1475767268-14379-3-git-send-email-tomeu.vizoso@collabora.com