drivers/xgi/z9s: Port Linux framebuffer initialization to coreboot

Add native XGI Z9s framebuffer support to coreboot
XGI initialization code largely taken from Linux 3.18.5

TEST: Booted KFSN4-DRE with XGI Volari Z9s into SeaBIOS
with SeaVGABIOS enabled.  Text appeared correctly on screen
and interaction with graphical comboot menu was successful.
However, Linux cleared the framebuffer on boot, rendering the
screen useless until Linux loaded its native xgifb driver.

Change-Id: I606a3892849fc578b0c4d74536aec0a0adef3be3
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: http://review.coreboot.org/8331
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/lib/edid.c

@@ -1489,6 +1489,7 @@ void set_vbe_mode_info_valid(struct edid *edid, uintptr_t fb_addr)
 	vbe_valid = 1;
 }
 
+#if IS_ENABLED(CONFIG_NATIVE_VGA_INIT_USE_EDID)
 int vbe_mode_info_valid(void)
 {
 	return vbe_valid;
@@ -1498,3 +1499,4 @@ void fill_lb_framebuffer(struct lb_framebuffer *framebuffer)
 {
 	*framebuffer = edid_fb;
 }
+#endif