drm/i915: Use drm_color_lut_size()

Avoid all the sizeof(drm_color_lut) business by using
drm_color_lut_size() to convert the blob length into
number of LUT entries.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180223192506.29992-6-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>