i915: Only allow 8 vertex texture units

i915: Only allow 8 vertex texture units

There's no reason to have more vertex texture units than fragment
texture units on this hardware.  Since increasing the default maximum
number of texture units from 16 to 32, this has triggered some segfault
in i915 driver.  There's probably some array or bitfield that isn't
properly sized now.  This really papers over the bug, but I don't think
I'll lose any sleep over that.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74071
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>