This adds dpm support for btc asics.  This includes:
- clockgating
- dynamic engine clock scaling
- dynamic memory clock scaling
- dynamic voltage scaling
- dynamic pcie gen1/gen2 switching (requires additional acpi support)

Set radeon.dpm=1 to enable.

v2: reduce stack usage
v3: attempt to fix state enable
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>