i915: Compute maximum number of verts using the actual batchbuffer size.

i915: Compute maximum number of verts using the actual batchbuffer size.

We were looking at the size of batch.map for how big the batchbuffer
was, but on 865 we just use a single-page batchbuffer due to hardware
limits.

v2: Removed check for sizeof map < bo->size, since that's always false.
    [change by anholt]
NOTE: This is a candidate for release branches.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=41495