drm: Introduce drm_color_lut_size()

Provide a small helper to convert the blob length in bytes
to the number of LUT entries.

v2: Add kerneldoc (Daniel)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180315152338.7248-1-ville.syrjala@linux.intel.com