asrock/b75pro3-m: fix the HDMI port

At the time I ported coreboot to this board, the HDMI port of my board
was broken, so I couldn't test libgfxinit on HDMI.

Change-Id: Ibb8a4ef55bd97fd2d0baacdbc72863c0985d3d76
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Reviewed-on: https://review.coreboot.org/25820
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/mainboard/asrock/b75pro3-m/gma-mainboard.ads

@@ -21,8 +21,8 @@ use HW.GFX.GMA.Display_Probing;
 private package GMA.Mainboard is
 
    ports : constant Port_List :=
-     (HDMI1,
-      HDMI2,
+     (HDMI1, -- mainboard DVI port
+      HDMI3, -- mainboard HDMI port
       Analog,
       others => Disabled);