drm/i915: Pass the crtc to the low level read_lut() funcs

The low level read_lut() functions don't need the entire crtc state
as they know exactly what they're reading. Just need to pass in the
crtc to get at the pipe. This now neatly mirrors the load_lut()
direction.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-10-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>