samples: vfio-mdev: fix error handing in mdpy_fb_probe()

Fix to return a negative error code from the framebuffer_alloc() error
handling case instead of 0, also release regions in some error handing
cases.

Fixes: cacade19

 ("sample: vfio mdev display - guest driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Message-Id: <20210520133641.1421378-1-weiyongjun1@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>