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

src/mesa/drivers/dri/i965/brw_context.c

@@ -599,6 +599,7 @@ brw_initialize_context_constants(struct brw_context *brw)
       ctx->Const.MaxClipPlanes = 8;
 
    ctx->Const.LowerTessLevel = true;
+   ctx->Const.LowerTESPatchVerticesIn = true;
    ctx->Const.PrimitiveRestartForPatches = true;
 
    ctx->Const.Program[MESA_SHADER_VERTEX].MaxNativeInstructions = 16 * 1024;