drm/i915/color: move pre-SKL gamma and CSC enable read to intel_color

Abstract the register access better. The DSPCNTR read could be moved to
either i9xx_plane.c or intel_color.c. The latter feels better, even if
the register is written in the former.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/33830dba5d69027469d976f6909740ccff8c7281.1692287501.git.jani.nikula@intel.com