drm: Only take crtc lock in get_gamma ioctl

We don't call into drivers at all here, this is enough. Also, we can
reduce the critical section a bit to simplify the code.
crtc->gamma_size is set up once at driver load and then invariant, so
also doesn't need any protection.
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-8-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_color_mgmt.c

@@ -295,19 +295,15 @@ int drm_mode_gamma_get_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EINVAL;
 
-	drm_modeset_lock_all(dev);
 	crtc = drm_crtc_find(dev, crtc_lut->crtc_id);
-	if (!crtc) {
-		ret = -ENOENT;
-		goto out;
-	}
+	if (!crtc)
+		return -ENOENT;
 
 	/* memcpy into gamma store */
-	if (crtc_lut->gamma_size != crtc->gamma_size) {
-		ret = -EINVAL;
-		goto out;
-	}
+	if (crtc_lut->gamma_size != crtc->gamma_size)
+		return -EINVAL;
 
+	drm_modeset_lock(&crtc->mutex, NULL);
 	size = crtc_lut->gamma_size * (sizeof(uint16_t));
 	r_base = crtc->gamma_store;
 	if (copy_to_user((void __user *)(unsigned long)crtc_lut->red, r_base, size)) {
@@ -327,6 +323,6 @@ int drm_mode_gamma_get_ioctl(struct drm_device *dev,
 		goto out;
 	}
 out:
-	drm_modeset_unlock_all(dev);
+	drm_modeset_unlock(&crtc->mutex);
 	return ret;
 }