st/mesa: disable line stippling if pattern is all 1's

st/mesa: disable line stippling if pattern is all 1's

The spec says:
  When disabled, it is as if theline stipple has its default value
(the default value being all 1's)

So treat pattern=0xffff as line stippling = off.

This improves performance in specviewperf13 snx lines tests.
For instance in the last test I get:
  * master: 260 fps, gpu-load: ~92%
  * with this commit: 280 fps, gpu-load: ~72%

(both tested with d60930c017 reverted)

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8105>