drm/i915/fbdev: Check for the framebuffer before use

If the fbdev probing fails, and in our error path we fail to clear the
dev_priv->fbdev, then we can try and use a dangling fbdev pointer, and
in particular a NULL fb. This could also happen in pathological cases
where we try to operate on the fbdev prior to it being probed.

Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468431285-28264-2-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>