drm/amdgpu/powerplay: fix smu7_get_memory_type for fiji

Fiji uses a different register than other smu7 asics, but
we already have this info in the base driver so just
use that.

Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>