drm: unify formatting for color management documentation

Other properties are documented with a colon character after the
property name. Consistently using a colon character allows the docs to
be machine-readable.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/7dhjUKqlDC0IiRRON0VhWnkuoxcRDLG4EFKzyhcEA@cp7-web-043.plabs.ch