zink: fix line-width calculation

zink: fix line-width calculation

There's two things that goes wrong in this code on some drivers:
1. Rounding off the line-width to granularity can push it outside the
   legal range.
2. A granularity of 0.0 results in NaN, because we divide by zero.

So let's make this code a bit more robust.

Acked-by: Jordan Justen <jordan.l.justen@intel.com>