Fix performance regression caused by 10069f3.

Apparently using one big overlay to disable `display' property
from (point) to (point-max) will slow to a crawl on large
files. We need to be more conservative with the outer overlay
placement.

* company.el(company-pseudo-tooltip-show): Progressively enlarge
  outer overlay until we cover the correct number of lines or
  (eobp) has been reached.