drm/radeon: cleanup properly if mmio mapping fails

commit 0cd9cb76 upstream.

If we fail to map the mmio BAR, skip driver tear down
that requires mmio.

Should fix:
https://bugzilla.kernel.org/show_bug.cgi?id=56541



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>