video: hx8357: Make IM pins optional

The IM pins of the HX8357 controller are used to define the interface
used to feed pixel stream to the LCD panel.

Most of the time, these pins are directly routed to either the ground or
the VCC to set their values.

Remove the need to assign GPIOs to these pins when we are in such a case.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>