s3fb: fix Virge/VX

Add memory size detection for Virge/VX and small delay in mode setting (same
as in X.org driver) to fix blank screen problem.

Also adjust DTPC position to fix garbled screen in some modes (tested that
this adjustment does not break other cards - at least Trio32, Trio64V+,
Trio64V2/DX, Virge, Virge/DX).

Tested on ELSA Winner 2000AVI/3D.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>