drm/amdgpu: Need to disable msix when unloading driver

For driver reload test, it will report "can't enable
MSI (MSI-X already enabled)".

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>