Change return value to int to propagate errors from gamma_set,
and remove start parameter. Updates always use the full size,
and some drivers even ignore the start parameter altogether.

This is needed for atomic drivers, where an atomic commit can
fail with -EINTR or -ENOMEM and should be restarted. This is already
and issue for drm_atomic_helper_legacy_set_gamma, which this patch
fixes up.

Changes since v1:
- Fix compiler warning. (Emil)
- Fix commit message (Daniel)

Cc: Alex Deucher <alexander.deucher@amd.com>
Acked-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Mathieu Larouche <mathieu.larouche@matrox.com>
Cc: Thierry Reding <treding@nvidia.com>
Acked-by: NPatrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Improve commit message a bit more, mention that this fixes
the helper.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/742944bc-9f41-1acb-df4f-0fd4c8a10168@linux.intel.com