drm/amdgpu: add contiguous flag in ucode bo create

Under VF environment, the ucode would be settled to the visible VRAM,
As it would be pinned to the visible VRAM, it's better to add
contiguous flag,otherwise it need to move gpu address during the pin
process. This movement is not necessary.

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