drm/nouveau/kms/nv50: request vblank events for commits that send completion events

This somehow fixes an issue where sync-to-vblank longer works correctly
after resume from suspend.

From a HW perspective, we don't need the IRQs turned on to be able to
detect flip completion, so it's assumed that this is required for the
voodoo in the core DRM vblank core.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_display.c

@@ -4052,6 +4052,11 @@ nv50_disp_atomic_commit_tail(struct drm_atomic_state *state)
 		}
 	}
 
+	for_each_crtc_in_state(state, crtc, crtc_state, i) {
+		if (crtc->state->event)
+			drm_crtc_vblank_get(crtc);
+	}
+
 	/* Update plane(s). */
 	for_each_plane_in_state(state, plane, plane_state, i) {
 		struct nv50_wndw_atom *asyw = nv50_wndw_atom(plane->state);
@@ -4101,6 +4106,7 @@ nv50_disp_atomic_commit_tail(struct drm_atomic_state *state)
 			drm_crtc_send_vblank_event(crtc, crtc->state->event);
 			spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
 			crtc->state->event = NULL;
+			drm_crtc_vblank_put(crtc);
 		}
 	}