staging/xgifb: remove unnecessary temp variable in XGIfb_mode_rate_to_ddata

Instead of subtracting one and then assign a different name and add 1
again we simply use HDE directly. HDE wasn't used directly before, so no
change in functionality.
Same applies to VDE.

-> now we can remove the variable with the very descriptive name E ;)

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