drm/amdgpu: move get_rev_id at first before load gpu_info firmware

Rev id is used for identifying Raven2 series of chips. So we would better to
initialize it at first.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>