drm/dp: add helpers for capture of frame CRCs

Adds helpers for starting and stopping capture of frame CRCs through the
DPCD. When capture is on, a worker waits for vblanks and retrieves the
frame CRC to put it in the queue on the CRTC that is using the
eDP connector, so it's passed to userspace.

v2: Reuse drm_crtc_wait_one_vblank
    Update locking, as drm_crtc_add_crc_entry now takes the lock

v3: Don't call wake_up_interruptible directly, that's now done in
    drm_crtc_add_crc_entry.

v4: Style fixes (Sean Paul)
    Reworked retry of CRC reads (Sean Paul)
    Flush worker after stopping CRC generationa (Sean Paul)

v5: Move back to make the retry explicitly once

v6: Set and use the drm_crtc backpointer (Sean Paul)

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303133936.14964-3-tomeu.vizoso@collabora.com