amdgpu: merge and cleanup amdgpu_bo_free

since bo_reference and bo_internal_free are
all only used by bo_free, so we just merge them
together

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>