staging/xgifb: Remove unnecessary fields of XGINew_DDRDRAM_TYPE{340, 20}

Since the first three entries in XGINew_DDRDRAM_TYPE{340,20} are never
used, we can simply remove them;
and instead of passing XGINew_DDRDRAM_TYPE with an index we can simply
pass the value directly to XGINew_SetDRAMSize20Reg.

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