staging/xgifb: Consolidate XGINew_SetDRAMSize{,20}Reg

This patch consolidates the almost identical functions
XGINew_SetDRAMSizeReg and XGINew_SetDRAMSize20Reg as they are
implemented identically except one division factor.

The changed factor is now reflected in the input data.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>