drm/nouveau/kms/tu102-: disable input lut when input is already FP16

On Turing, an input LUT is required to transform inputs in fixed-point
formats to FP16 for the internal display pipe.  We provide an identity
mapping whenever a window is enabled for this reason.

HW has error checks to ensure when the input is already FP16, that the
input LUT is also disabled.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>