[Intel-gfx] Enable tiling by default for big perf wins on i945

[Chris Wilson]

We had a request just before the 2.9 release to fix the performance
problem with tfp with large >1020 pixmaps: bug #25385

  http://bugs.freedesktop.org/show_bug.cgi?id=25375

The patch is itself fairly simple, a minor change to enable TILING_X by
default and remove it if the pixmap is small in comparison to the fence
size. However, after enabling tiling for the glyph cache there were
immediate reports of glyph corruption following s&r, vt switching and
memory pressure:

  http://bugs.freedesktop.org/show_bug.cgi?id=25406

I believe that we have resolved all the issues since then -- as the glyph
cache is tiled once more and nobody has been complaining about broken
glyphs. [Except for the first few months nobody could even run 2.10 due to
the SET_TILING brokenness...] So I think this is a relatively safe patch,
but would appreciate a second or third opinion before merging it at this
point.
-ickle