drm/drm_debugfs_crc.c: Document that .verify_crc_source vfunc is required for enabling CRC support.

drm_debugfs_crtc_crc_add() function checks that both .set_crc_source and
.verify_crc_source hooks are provided before enabling debugfs support for
reading per-frame CRC data. Make that explicit in the documentation.

Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703150330.21992-1-Liviu.Dudau@arm.com