drm/doc: move color management functions under CRTC section

Move drm_color_mgmt function reference from the KMS properties
section to the CRTC abstraction section. This makes the KMS
properties section more readable for user-space developers.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201216202222.48146-5-contact@emersion.fr