drm/vc4: Drop of_gpio header

This driver includes the deprecated OF GPIO header <linux/of_gpio.h>
yet fail to use symbols from it, so drop the include.

Cc: Emma Anholt <emma@anholt.net>
Cc: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220812205746.609107-6-mairacanal@riseup.net