drm/amdgpu: fix 64bit BAR detection

Windows added by the BIOS are not marked as 64bit because they are
usually not changeable anyway.

This fixes large BAR support on my new Ryzen build system.

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>