drm/amdgpu: add helper func for psp ring cmd submission

Except for ring wptr update, the psp ring cmd submission
function shouldn't be IP specific one. Create a common
helper function to be shared for all the ASICs.

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