staging/xgifb: Move duplicated code for dram to helper function

XGINew_SetDRAMDefaultRegister340 uses the same code fragment 4 times
with only a slight variation each time.
-> Move this code to a helper function - this saves some lines and
~450bytes in the .o / .ko

Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>