nir: Fix two-sided color lowering on Panfrost

Fixes the two-sided-lighting and vertex-program-two-side piglit tests on Panfrost.

No other drivers disable both PIPE_CAP_TGSI_FS_FACE_IS_INTEGER_SYSVAL and PIPE_CAP_TWO_SIDED_COLOR, so there should be no changes for other drivers.

It's only six months since I first reported this issue...