s3fb: add DDC support

Add I2C support for the DDC bus and also default mode initialization by
reading monitor EDID to the s3fb driver.

Tested on Trio64V+ (2 cards), Trio64V2/DX, Virge (3 cards),
Virge/DX (3 cards), Virge/GX2, Trio3D/2X (4 cards), Trio3D.

Will probably not work on Trio32 - my 2 cards have DDC support in BIOS that
looks different from the other cards but the DDC pins on the VGA connector
are not connected.

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