staging: xgifb: fix multiple line dereference

This patch fixes the checkpatch.pl warning:

WARNING: Avoid multiple line dereference - prefer'XGI330_EModeIDTable[ModeIdIndex].Ext_ModeFlag'

Signed-off-by: Aastha Gupta <aastha.gupta4104@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>