backends/drm: Prefer egl import mode

The egl import mode ensures that there's a local buffer, which is
preferred as it minimizes the number of data transfers over PCIe.

With dmabuf, it's unclear what the driver will do. But the main takeaway
from discussion with mesa developers is that it's undesired for
gbm_bo_import() to migrate or perform data transfers behind the user's
back, it should be done explicitly.