drm/amdgpu: add psp funcs for ring write pointer read/write

The ring write pointer regsiter update is the only part that
is IP specific ones in psp_cmd_submit function.

Add two callbacks for wptr read/write so that we unify the
psp_cmd_submit function 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>