drm/amdgpu/psp: move shared buffer frees into single function

So we can properly clean up if any of the TAs or TMR fails
to properly initialize or terminate.  This avoids any
memory leaks in the error case.

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