drm/atomic-helper: Remove legacy backoff hack from gamma_set

Another one knocked down.

With this we can also remove the temporary hack in the gamma_set
ioctl.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-14-daniel.vetter@ffwll.ch