i965: Use a uniform for gl_PatchVerticesIn in the TES.

i965: Use a uniform for gl_PatchVerticesIn in the TES.

Fixes three GL44-CTS.tessellation_shader subtests:
- max_patch_vertices
- single.max_patch_vertices
- tessellation_control_to_tessellation_evaluation.gl_PatchVerticesIn

These use gl_PatchVerticesIn in the TES, but don't link against a
TCS (which would allow the linker to lower it to a constant).  We had
no handling for the system value in the backend, so it would just
assert fail.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Cc: mesa-stable@lists.freedesktop.org