drm/amdgpu: SHADOW and VRAM_CONTIGUOUS flags shouldn't be used by userspace

The shadow handling isn't implemented completely for userspace BOs and
the kernel sets the VRAM_CONTIGUOUS as necessary.

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