drm/omap: Fix missing includes

With certain kernel config options many omapdrm files fail to compile
due to missing include of linux/gpio/consumer.h and linux/of.h.

This patch adds those includes.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>