drm/core: Change declaration for gamma_set.

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: Alex 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: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Improve commit message a bit more, mention that this fixes
the helper.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/742944bc-9f41-1acb-df4f-0fd4c8a10168@linux.intel.com

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -175,20 +175,22 @@ vc4_crtc_lut_load(struct drm_crtc *crtc)
 		HVS_WRITE(SCALER_GAMDATA, vc4_crtc->lut_b[i]);
 }
 
-static void
+static int
 vc4_crtc_gamma_set(struct drm_crtc *crtc, u16 *r, u16 *g, u16 *b,
-		   uint32_t start, uint32_t size)
+		   uint32_t size)
 {
 	struct vc4_crtc *vc4_crtc = to_vc4_crtc(crtc);
 	u32 i;
 
-	for (i = start; i < start + size; i++) {
+	for (i = 0; i < size; i++) {
 		vc4_crtc->lut_r[i] = r[i] >> 8;
 		vc4_crtc->lut_g[i] = g[i] >> 8;
 		vc4_crtc->lut_b[i] = b[i] >> 8;
 	}
 
 	vc4_crtc_lut_load(crtc);
+
+	return 0;
 }
 
 static u32 vc4_get_fifo_full_level(u32 format)