xrandr: use 1/gamma to compute gamma-correction

xrandr: use 1/gamma to compute gamma-correction

To compute a gamma *correction* lookup table, use the specified gamma
value as the divisor in (1.0/gamma).  This matches the semantics of
xgamma(1) and the "gamma-value" and "{red,green,blue}-gamma" xorg.conf(5)
options.

For more details, see:
http://www.poynton.com/PDFs/TIDV/Gamma.pdf (Gamma in computer graphics, page 17)
http://cgit.freedesktop.org/xorg/xserver/tree/hw/xfree86/common/xf86cmap.c:ComputeGamma()

Signed-off-by: Andy Ritger <aritger@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>