drm/amdgpu: add determine passthrough under arm64

add determine for passthrough mode under arm64 by reading
CurrentEL register

v2: squash in warning fix (Alex)

Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>