drm/radeon: add bapm module parameter

Add a module paramter to enable bapm on APUs.  It's disabled
by default on certain APUs due to stability issues.  This
option makes it easier to test and to enable it on systems that
are stable.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=81021



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org