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/sandybridge/gma.c

@@ -24,6 +24,7 @@
 #include <device/pci_ops.h>
 #include <cpu/x86/msr.h>
 #include <cpu/x86/mtrr.h>
+#include <drivers/intel/gma/libgfxinit.h>
 #include <southbridge/intel/bd82x6x/nvs.h>
 #include <drivers/intel/gma/opregion.h>
 #include <cbmem.h>