i965: lower gl_PatchVerticesIn to a uniform

i965: lower gl_PatchVerticesIn to a uniform

We want this here instead of nir_lower_system_values because for
Vulkan we don't want this lowering to take place.

v2: do not try to handle it as a system value directly for the SPIR-V
    path. In GL we rather handle it as a uniform like we do for the
    GLSL path (Jason).

v3: do this also for the TessEval stage (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>