drm: Add DRM_MODESET_LOCK_BEGIN/END helpers

This patch adds a couple of helpers to remove the boilerplate involved
in grabbing all of the modeset locks.

I've also converted the obvious cases in drm core to use the helpers.

The only remaining instance of drm_modeset_lock_all_ctx() is in
drm_framebuffer. It's complicated by the state clear that occurs on
deadlock. ATM, there's no way to inject code in the deadlock path with
the helpers, so it's unfit for conversion.

Changes in v2:
- Relocate ret argument to the end of the list (Daniel)
- Incorporate Daniel's doc suggestions (Daniel)

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129150423.239081-4-sean@poorly.run

drivers/gpu/drm/drm_plane.c

@@ -767,11 +767,8 @@ static int setplane_internal(struct drm_plane *plane,
 	struct drm_modeset_acquire_ctx ctx;
 	int ret;
 
-	drm_modeset_acquire_init(&ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE);
-retry:
-	ret = drm_modeset_lock_all_ctx(plane->dev, &ctx);
-	if (ret)
-		goto fail;
+	DRM_MODESET_LOCK_ALL_BEGIN(plane->dev, ctx,
+				   DRM_MODESET_ACQUIRE_INTERRUPTIBLE, ret);
 
 	if (drm_drv_uses_atomic_modeset(plane->dev))
 		ret = __setplane_atomic(plane, crtc, fb,
@@ -782,14 +779,7 @@ retry:
 					  crtc_x, crtc_y, crtc_w, crtc_h,
 					  src_x, src_y, src_w, src_h, &ctx);
 
-fail:
-	if (ret == -EDEADLK) {
-		ret = drm_modeset_backoff(&ctx);
-		if (!ret)
-			goto retry;
-	}
-	drm_modeset_drop_locks(&ctx);
-	drm_modeset_acquire_fini(&ctx);
+	DRM_MODESET_LOCK_ALL_END(ctx, ret);
 
 	return ret;
 }