soc/intel/common/gfx: rename and guard graphics_soc_init()

Rename to graphics_soc_panel_init, to more accurately convey
operations performed by the function. Guard execution so we
don't attempt to reconfigure the panel after FSP has already
done so.

This fixes FSP/GOP display init on APL/GLK, which was broken by
attempting to configure the panel after FSP had already done so.

Change-Id: I8e68a16b2efb59965077735578b1cc6ffd5a58f0
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48884
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/skylake/graphics.c

@@ -12,7 +12,7 @@
 #include <soc/systemagent.h>
 #include <types.h>
 
-void graphics_soc_init(struct device *dev)
+void graphics_soc_panel_init(struct device *dev)
 {
 	struct soc_intel_skylake_config *conf = config_of(dev);
 	struct resource *mmio_res;