drm/atomic: convert drm_atomic_get_{old, new}_colorop_state() into proper functions

drm/atomic: convert drm_atomic_get_{old, new}_colorop_state() into proper functions

There is no real reason to include drm_colorop.h from drm_atomic.h, as
drm_atomic_get_{old,new}_colorop_state() have no real reason to be
static inline.

Convert the static inlines to proper functions, and drop the include to
reduce the include dependencies and improve data hiding.

v2: Fix vkms build failures (Alex)

Fixes: cfc27680ee20 ("drm/colorop: Introduce new drm_colorop mode object")
Cc: Simon Ser <contact@emersion.fr>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Daniel Stone <daniels@collabora.com>
Cc: Melissa Wen <mwen@igalia.com>
Cc: Sebastian Wick <sebastian.wick@redhat.com>
Cc: Alex Hung <alex.hung@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Link: https://patch.msgid.link/20251219114939.1069851-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>