[media] vsp1: make vsp1_drm_frame_end static

As reported by smatch:
	drivers/media/platform/vsp1/vsp1_drm.c:39:6: warning: no previous prototype for 'vsp1_drm_frame_end' [-Wmissing-prototypes]
	 void vsp1_drm_frame_end(struct vsp1_pipeline *pipe)

Fixes: ef9621bc

 ("[media] v4l: vsp1: Store the display list manager in the WPF")
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>