i965/gs: Avoid DW * DW mul

i965/gs: Avoid DW * DW mul

The GS has an interesting use for mul. Because the GS can emit multiple
vertices per input vertex, and it also has a unique count at the top of the URB
payload, the GS unit needs to be able to dynamically specify URB write offsets
(relative to the global offset). The documentation in the function has a very
good explanation from Paul on the mechanics.

This fixes around 2000 piglit tests on BSW.

v2:
Reworded commit message (Ben) no mention of CHV (Matt)
Change SHRT_MAX to USHRT_MAX (Ken, and Matt)
Update comment in code to reflect the use of UW (Ben)
Add Gen7+ assertion for the relevant GS code, since it won't work on Gen6- (Ken)
Drop the bogus hunk in emit_control_data_bits() (Ken)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84777 (with many dupes)
Cc: "10.4 10.3 10.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>