drm/qxl: Don't set a gamma table size

qxl doesn't have any functions for setting the gamma table, so this is
completely defunct.

Not nice to lie to userspace, so let's stop!

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1459331485-28376-10-git-send-email-daniel.vetter@ffwll.ch