drm/amdgpu/psp: add a mutex to protect access to the psp ring

drm/amdgpu/psp: add a mutex to protect access to the psp ring

We need to serialize access to the psp ring if there are multiple
callers at runtime.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>