i965: Fix a mistake from porting the URB allocation code to arrays.

i965: Fix a mistake from porting the URB allocation code to arrays.

Commit 6d416bcd846a49414f210cd761789156c37a7b3e (i965: Use arrays in
Gen7+ URB code.) introduced a regression which caused us to fail to
allocate all of our URB space.

   -         total_wants -= ds_wants;
   +         total_wants -= additional;

The new line should have been total_wants -= wants[i].

Fixes a large performance regression in TessMark.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98815
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>