vfio/display: Fix error path in call site of ramfb_setup()

vfio/display: Fix error path in call site of ramfb_setup()

vfio_display_dmabuf_init() and vfio_display_region_init() calls
ramfb_setup() without checking its return value.

So we may run into a situation that vfio_display_probe() succeed
but errp is set. This is risky and may lead to assert failure in
error_setv().

Cc: Gerd Hoffmann <kraxel@redhat.com>
Fixes: b290659fc3d ("hw/vfio/display: add ramfb support")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>