drm/amdgpu: force fbdev into vram

We set the fb smem pointer to the offset into the BAR, so keep
the fbdev bo in vram.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=207581
Fixes: 6c8d74ca

 ("drm/amdgpu: Enable scatter gather display support")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org