backends/drm: fix some VM things
commit 1: xrgb is effectively universally supported, other formats may not be. In order to make sure things work everywhere, assume only xrgb is supported.
commit 2: don't assume that we should only use modifiers if the IN_FORMATS
property is available. Instead, assume we should use DRM_FORMAT_MOD_LINEAR
.
Edited by Xaver Hugl