i965/curbe: Look at SYSTEM_VALUE_FRAG_COORD instead of VARYING_SLOT_POS
When transitioning gl_FragCoord over to a system value, we missed one instance of VARYING_SLOT_POS in i965. As of this commit, i965 has no references to VARYING_SLOT_POS.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111263 Fixes: 4bb6e681 "intel: Use a system value for gl_FragCoord"