drm/amdgpu: add supports_baco callback for soc15 asics. (v2)

BACO - Bus Active, Chip Off

Check the BACO capabilities from the powerplay table.

v2: drop unrelated struct cleanup

Reviewed-by: Evan Quan <evan.quan@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>