drm/amdgpu: fix SI UVD firmware validate resume fail

The SI UVD firmware validate key is stored at the end of firmware,
which is changed during resume while playing video. So get the key
at sw_init and store it for fw validate using.

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>