drm/radeon/dpm: handle bapm on kb/kv

bapm is a power management feature for handling the
power budget between the CPU and GPU on APUs.  This
patch adds support for enabling or disabling it.
For now disable it by default.  Enabling it properly
requires quite a bit more work and will be addressed
in a separate patch.

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