drm/amd/pm: Add interface to get FW private buffer

v1: Add new interface to get FW private buffer details
v2: Drop domain check
v3: Use amdgpu_bo_kmap to get cpu address

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>