drm/fb-helper: generic: Don't take module ref for fbcon

It's now safe to let fbcon unbind automatically on fbdev unregister.
The crash problem was fixed in commit 2122b405


("fbdev: fbcon: Fix unregister crash when more than one framebuffer")

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190210131039.52664-13-noralf@tronnes.org