plat: imx8mm: fix the gpumix vpumix power down

the GPU/VPU mix power off is skip in previous code,
so correct to make sure GPU/VPU mix is actually power
off.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
(cherry picked from commit 102d081b3f9ac569cdff760ac7c7f7c402c3f5fe)
1 file changed