drm/amdgpu: Restore msix after FLR

After FLR, the msix will be cleared, so need to re-enable it.

Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Signed-off-by: Emily.Deng <Emily.Deng@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>