drm/amdgpu: fix race between pstate and remote buffer map

Vega20 arbitrates pstate at hive level and not device level. Last peer to
remote buffer unmap could drop P-State while another process is still
remote buffer mapped.

With this fix, P-States still needs to be disabled for now as SMU bug
was discovered on synchronous P2P transfers.  This should be fixed in the
next FW update.

Signed-off-by: Jonathan Kim <Jonathan.Kim@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>