RFC: kms_swrast: fix mapping for vgem+vkms interop

This is RFC because I am not sure if this is a supported usecase for kms_swrast, and I still need to test it.

According to https://lists.freedesktop.org/archives/dri-devel/2023-January/385397.html, vgem+vkms can work except kms_swrast should not rely on DRM_IOCTL_MODE_MAP_DUMB to work on imported dma-bufs.