weston-init: replace deprecated/disabled fbdev with drm backend

drm will default to opengl rendering by default; if this is
problematic, --use-pixman option may help, apply only where
it's proven necessary.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>