s3fb: fix Virge/GX2

Fix Virge/GX2 support in s3fb:
- fix IDs: 86C357 is GX2, 86C359 is GX2+, 86C356 probably does not exist
- add memory size detection
- drive it the same way as Trio3D/2X

The original IDs most likely came from S3 website which claims that:
- 356 is Virge/GX2 with ID=8A10, driver included in Windows 2K, XP
- 357 is Virge/GX2+ with ID=8A11, driver included in Windows ME
- 359 is Virge/GX2+ with ID=8A12, driver included in Windows ME
but:
- google search for 86C356 only reveals references to Trio3D (probably
because of a typo - Trio3D is 86C365)
- my card is clearly marked as 86C357, Virge/GX2 and has ID=8A10
- there is no driver for IDs 8A11 and 8A12 in Windows ME
- there is a driver for ID 8A10 in Windows ME that says it's GX2 (357)

Tested with #9 Reality 334 (86C357 Virge/GX2, ID=0x8A10).

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>