drm/amdgpu: Fix firmware UCODE_ID_STORAGE issue (v2)

In Tonga's virtualization environment, for firmware UCODE_ID_STORAGE,
there is no actual firmware data, but we still need alloc a BO and
tell the BO's mc address to HW, or world switch will hang on VFs.

v2: fix coding style (Alex)

Signed-off-by: Trigger Huang <trigger.huang@amd.com>
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>