drm/i915: Disable plane gamma in SKL+ sprite planes

The plane gamma tables are never programmed, so just disable it, like it
is done for the primary plane.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485429865-10687-2-git-send-email-ander.conselvan.de.oliveira@intel.com