anv/pipeline: allow more than 16 FS inputs

A fragment shader can have more than 16 inputs, so SBE emission should
deal with all of them.

This fixes dEQP-VK.pipeline.max_varyings.*

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Ivan Briano <ivan.briano@intel.com>
Tested-by: Marge Bot <mesa/mesa!2010>
Part-of: <mesa/mesa!2010>