i965: Fix the SF Vertex URB Read Length calculation for Gen7 platforms.

Ivybridge doesn't appear to have the same errata as Sandybridge; no
corruption was observed by setting it to more than the minimal correct
value.  It's possible that we were simply lucky, since the URB entries
are 1024-bit on Ivybridge vs. 512-bit Sandybridge.  Or perhaps the
underlying hardware issue is fixed.

Either way, we may as well program the minimum value since it's now
readily available, likely to be more efficient, and possibly more
correct.

v2: Use GEN7_SBE_* defines rather than GEN6_SF_*.  (A copy and paste
    mistake.)  They're the same, but using the right names is better.

NOTE: This is a candidate for all stable branches.
Reviewed-by: Paul Berry <[email protected]>
Signed-off-by: Kenneth Graunke <[email protected]>

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