drm/amdgpu/umsch: declare umsch firmware

Needed to be properly picked up for the initrd, etc.

Fixes: 3488c79b ("drm/amdgpu: add initial support for UMSCH")
Reviewed-by: Saleemkhan Jamadar <saleemkhan.jamadar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Lang Yu <Lang.Yu@amd.com>