iris: Ignore line stipple information if it's disabled

iris: Ignore line stipple information if it's disabled

The line stipple pattern and factor only matter if line stippling is
actually enabled.  Otherwise, we can safely ignore it.

PBO upload may give us zero for line stipple information, while normal
drawing tends to give us an actual stipple pattern such as 0xffff.  This
was causing us to flag IRIS_DIRTY_LINE_STIPPLE way too often, leading to
useless 3DSTATE_LINE_STIPPLE commands, which are non-pipelined and thus
very expensive.

Improves performance in Manhattan 3.0 on Skylake GT4e by
0.149261% +/- 0.0380796% (n=210).  On an Icelake 8x8 with the GPU
frequency locked at 700Mhz, improves by 0.423756% +/- 0.222843% (n=3).