egl:fbdev: lazily allocate buffers.

No point in reallocating the buffers 3-4 times before we start drawing.
Instead, delay creating them until the first dequeu.

This incidientally fixes an issue where we fail to allocate a third
buffer beceause setBufferCount gets called after queueBuffer().
Because one buffer is locked by the fb device (due to the post())
we end up with only being able to reallocate 2 buffers this time
and the application continues with 2 buffers rather than 3.