drm: Constify drm_color_lut_check()

drm: Constify drm_color_lut_check()

drm_color_lut_check() doens't modify the passed in blob so
let's make it const.

Also s/uint32_t/u32/ while at it.

v2: Reduce line wraps (Sam)

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129170609.5718-1-ville.syrjala@linux.intel.com
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>