drm/i915/glk: Load the degamma LUT even in legacy gamma mode

In Geminilake, the degamma table is enabled or disabled by the pipe CSC
enable bit, so its active even when running in the legacy gamma mode.
So always set sane values for that table, since the default value is all
zeroes.

This fixes blank screens after a suspend/resume cycle while legacy gamma
is in use.

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/20170217120630.6143-2-ander.conselvan.de.oliveira@intel.com