drm/qxl: enables gem prime helpers for qxl using dummy driver callbacks

As there should not be any other virtual device that might share buffers,
the callbacks remain empty stubs. Still prime can be used to transfer buffers
between processes that use qxl.

Signed-off-by: Andreas Pokorny <andreas.pokorny@canonical.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>