i.MX Framebuffer: remove gpio setup function

Remove the gpio mux setup function from i.MX framebuffer driver.
This function is platform specific and thus should be done by
the board setup. As there are currently no in-kernel users
of this driver we do not break anything.

Acked-by: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>