nir/lower_tex: Fix minor error in YUV color conversion matrix

nir/lower_tex: Fix minor error in YUV color conversion matrix

The matrix used for YCbCr to RGB is listed in:

    https://en.wikipedia.org/wiki/YCbCr

There was an error in converting the offsets from integers to unorm
values: 0.0625=16/256 should be 16.0/255,and 0.5=128.0/256 should be
128.0/255.  With this fix, the CSC result is bit aligned with wikipedia's
conversion result and FFMPeg's result.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100854
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>