drm/radeon: cleanup properly if mmio mapping fails

commit 0cd9cb76ae26a19df21abc6f94f5fff141e689c7 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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Weng Meiling <wengmeiling.weng@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>