drm/uapi: document kernel capabilities

Document all of the DRM_CAP_* defines.

v2 (Pekka):
- Describe what the bit depth is
- Expand on preferred dumb buffer memory access patterns
- Explain what a PRIME buffer is
- Mention DRM_IOCTL_PRIME_FD_TO_HANDLE and DRM_IOCTL_PRIME_HANDLE_TO_FD
- Explicitly reference CLOCK_REALTIME and CLOCK_MONOTONIC
- Make it clear DRM_CAP_CRTC_IN_VBLANK_EVENT applies to both DRM_EVENT_VBLANK
  and DRM_EVENT_FLIP_COMPLETE

v3 (Daniel):
- Specify kernel versions for caps that don't depend on drivers
- Make it clear dumb buffers caps are only about dumb buffers

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210308123421.747836-1-contact@emersion.fr