Send modifiers to client despite udev rule

Two important commits:

We disable modifiers for two reasons: an udev rule saying so, or the
lack of a working drmModeAddFB2(). However, to the users, this is not
granular enough. While the current user, whether to enable modifiers in
MetaRendererNative, doesn't need more granularity, we want to send
modifiers to Wayland clients even if the onscreen framebuffers should
still be allocated without modifiers.

Prepare for differentiating between how Wayland DMA buffers work and how
onscreen buffer allocation work, first prepare by separating the
relevant device flags.

and

wayland/dma-buf: Always send modifiers if AddFB2() is supported

To Wayland clients, it doesn't matter how we configure our onscreen
buffers, since they don't necessarily have the same bandwidth issues
related to mode setting, whichis the primary reason why we disable
modifiers using the udev rule, so simply check whether importing with
modifiers will work at all and advertise modifiers if so is the case.

This might help avoid issues using legacy non-modifiers path in drivers.