st/mesa: disable line stippling if pattern is all 1's
The spec says: When disabled, it is as if the line 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 d60930c0 reverted)