drm/atmel-hlcdc: Rework the fbdev creation logic

Now that we wait for DRM panels to be available before registering the
DRM device (returning -EPROBE_DEFER if the panel has not been probed
yet), we no longer need to put the fbdev creation code in
->output_poll_changed().

This removes the 10 secs delay between DRM dev registration and fbdev
creation (polling period = 10 seconds).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Alex Vazquez <avazquez.dev@gmail.com>