drivers/intel/gma: Put gma_gfxinit() into its own header

The current header `i915.h` is too invasive to be used everywhere where
we want to use *libgfxinit*.

Change-Id: Iba57256d536e301e598d98182448d2daa1bf9a89
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/20112
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/northbridge/intel/nehalem/gma.c

@@ -27,6 +27,7 @@
 #include <drivers/intel/gma/edid.h>
 #include <drivers/intel/gma/i915.h>
 #include <drivers/intel/gma/intel_bios.h>
+#include <drivers/intel/gma/libgfxinit.h>
 #include <pc80/vga.h>
 #include <pc80/vga_io.h>
 #include <southbridge/intel/ibexpeak/nvs.h>