nir,panfrost,agx: Fix driver PIXEL_COORD_INTEGER setting and drop workaround.
nir_lower_frag_coord_to_pixel_coord was adding .5 to work around that the drivers were mistakenly setting PIXEL_COORD_HALF_INTEGER. With the setting corrected, the GL frontend handles it appropriately (instead of subtracting half in the frontend for ARB_fragment_coord_conventions integer setting and then adding the half back here), and makes the pass reusable from Intel.
Cherry-pick from Emma's irk tree.