xwayland/glamor/gbm: make wl_drm optional

Build on top of !818 (merged) to use linux-dmabuf to grab the main device when wl_drm is unavailable. Fixes Xwayland glamor on top of latest wlroots commit which has dropped wl_drm support (wlroots/wlroots!4397 (merged)).

• Compared to !996, it's rebased on top of master.
• Compared to !968, it uses the linux-dmabuf main device to replace everything wl_drm.device was used for (not just one function).
• This MR doesn't handle any main device change, it merely removes Xwayland's hard dependency on wl_drm.