iris: Disable VF cache partial address workaround on Gen11+

iris: Disable VF cache partial address workaround on Gen11+

The vertex cache uses the full 48-bit address on Gen11+.  See the
documentation for 3DSTATE_VERTEX_BUFFERS, which describes the
workaround and lists it as pre-Icelake.

Interestingly, the docs don't mention index buffers as needing a
workaround at all.  So either we've been overzealous, or the docs
never got updated to record that.  Which begs the question of whether
the issue there was fixed, if there was one...

Cuts 40% of the PIPE_CONTROLs from Civilization VI's benchmark; appears
that it improves performance by about 1-2% on Icelake 8x8 (not frequency
locked).