xf86drm: warn about GEM handle reference counting

Users need to be careful when using drmPrimeHandleToFD or
drmPrimeFDToHandle directly. Mention GBM as a solution.

See [1] for an example mistake.

[1]: https://gitlab.freedesktop.org/drm/nouveau/-/issues/43#note_772661



Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>