[media] vsp1_drm.h: add missing prototypes

drivers/media/platform/vsp1/vsp1_drm.c:47:5: warning: no previous prototype for 'vsp1_du_init' [-Wmissing-prototypes]
 int vsp1_du_init(struct device *dev)
     ^
drivers/media/platform/vsp1/vsp1_drm.c:76:5: warning: no previous prototype for 'vsp1_du_setup_lif' [-Wmissing-prototypes]
 int vsp1_du_setup_lif(struct device *dev, unsigned int width,
     ^
drivers/media/platform/vsp1/vsp1_drm.c:221:6: warning: no previous prototype for 'vsp1_du_atomic_begin' [-Wmissing-prototypes]
 void vsp1_du_atomic_begin(struct device *dev)
      ^
drivers/media/platform/vsp1/vsp1_drm.c:273:5: warning: no previous prototype for 'vsp1_du_atomic_update' [-Wmissing-prototypes]
 int vsp1_du_atomic_update(struct device *dev, unsigned int rpf_index,
     ^
drivers/media/platform/vsp1/vsp1_drm.c:451:6: warning: no previous prototype for 'vsp1_du_atomic_flush' [-Wmissing-prototypes]
 void vsp1_du_atomic_flush(struct device *dev)
      ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>