drm/fb-helper: Add module option to disable fbdev emulation

Faster than recompiling.

Note that restore_fbdev_mode_unlocked is a bit special and the only
one which returns an error code when fbdev isn't there - i915 needs
that one to not fall over with some additional fbcon related restore
code. Everyone else just ignores the return value or only prints a
DRM_DEBUG level message.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>