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 <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Daniel Vetter <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
(cherry picked from commit 6bc265424df02f8162f4a17a37e4982e1c64460e)
Signed-off-by: Jani Nikula <[email protected]>

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