drm/amd/pm: bug fix for the runtime pm BACO

In some systems only MACO is supported. This is to fix the problem
that runtime pm is enabled but BACO is not supported. MACO will be
handled seperately.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Jack Gui <Jack.Gui@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org