intel/brw: Skip per-primitive inputs when computing flat input mask
The per-primitive have their own separate section in the FS thread payload, and are not considered when setting the mask in 3STATE_SBE's ConstantInterpolationEnable.
This is also consistent with what is done for brw_interp_reg().
Fixes
- dEQP-VK.mesh_shader.ext.misc.clip_geom_provoking_last
- dEQP-VK.mesh_shader.ext.misc.clip_geom_and_task_shader_provoking_last
Backport-to: 24.2