drm/cirrus: fix leaky driver load error handling

Before this patch, cirrus_device_init could have failed while
cirrus_mm_init succeeded and the driver would have reported overall
success on load. This patch causes cirrus_device_init to return on
the first error encountered.

Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>