mb/asus/p8z77-m_pro: Remove incorrect gfx_uma_size options

Values here above 512M are:
1. Inconsistent with Intel IVB datasheet vol.2, document #326765;
2. Apparently not properly supported by nb/intel/sandybridge.

Take them out until a fix can be implemented.

Change-Id: I6183f447af2816d00c9f6d78329113cd9c584191
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/88293
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asus/p8x7x-series/variants/p8z77-m_pro/cmos.layout

@@ -113,21 +113,6 @@ enumerations
 7	13	448M
 7	14	480M
 7	15	512M
-7	16	544M
-7	17	576M
-7	18	608M
-7	19	640M
-7	20	672M
-7	21	704M
-7	22	736M
-7	23	768M
-7	24	800M
-7	25	832M
-7	26	864M
-7	27	896M
-7	28	928M
-7	29	960M
-7	30	992M
 
 # usb3_mode
 # Disable = Use the port always as USB 2.0 for compatibility