media: rga: make some functions static

drivers/media/platform/rockchip/rga/rga-hw.c:383:6: warning: no previous prototype for 'rga_cmd_set' [-Wmissing-prototypes]
 void rga_cmd_set(struct rga_ctx *ctx)
      ^~~~~~~~~~~
drivers/media/platform/rockchip/rga/rga.c:359:17: warning: no previous prototype for 'rga_fmt_find' [-Wmissing-prototypes]
 struct rga_fmt *rga_fmt_find(struct v4l2_format *f)
                 ^~~~~~~~~~~~

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