staging/xgifb: Remove duplicated code in loops.

Instead of calling the same function three times in a loop, multiply the
loop counter by three.
And since the value in DAC_TEST_PARMS is always the same we can use the
value directly.

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