drm/radeon/TN: only enable bapm on MSI systems

commit 730a336c upstream.

There still seem to be stability problems with other systems.

Bug:
https://bugs.freedesktop.org/show_bug.cgi?id=72921



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>