xrandr: fix gamma == 1.0 && sigbits != 8

xrandr: fix gamma == 1.0 && sigbits != 8

The gamma-correction lookup table managed through XRR[GS]etCrtcGamma is
2^n in size, where 'n' is the number of significant bits in the X Color.
Each element in the gamma-correction lookup table is a 16:16:16 X Color
(i.e., in the range [0,65536) ).  The significant bits of each component
of each element in the lookup table are programmed into the hardware
lookup table.  Meaningful values in the gamma-correction lookup table
are thus in the range [0,2^sigbits), where all values are shifted into
the MSBs (i.e., left shifted by (16 - sigificant bits)).

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