drv/intel/gma/i915: Get rid of unused function prototype

The function prototype isn't used any more, remove it.

Change-Id: Ie5bd4e4ec8f28bc0768d5427cf734ef77855a15e
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/20545
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/drivers/intel/gma/i915.h

@@ -101,8 +101,6 @@ drivers_intel_gma_displays_ssdt_generate(const struct i915_gpu_controller_info *
 const struct i915_gpu_controller_info *
 intel_gma_get_controller_info(void);
 
-void *igd_make_opregion(void);
-
 /* vbt.c */
 struct device;
 void