drm/radeon/dpm: handle bapm on trinity

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.

This patch fixes hangs on boot on certain trinity
laptops when the system is on battery power.

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