drm/i915/glk: Enable pipe CSC

Now that the pre-csc degamma table is set up correctly in Geminilake,
pipe CSC can be enabled without causing a black screen.

v2: Rebase.
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/20170217120630.6143-3-ander.conselvan.de.oliveira@intel.com