drm/i915/selftests: Tighten the timeout testing for partial mmaps

Currently, if there is time remaining before the start of the loop, we
do one full iteration over many possible different chunks within the
object. A full loop may take 50+s (depending on speed of indirect GTT
mmapings) and we try separately with LINEAR, X and Y -- at which point
igt times out. If we check more frequently, we will interrupt the loop
upon our timeout -- it is hard to argue for as this significantly reduces
the test coverage as we dramatically reduce the runtime. In practical
terms, the coverage we should prioritise is in using different fence
setups, forcing verification of the tile row computations over the
current preference of checking extracting chunks. Though the exhaustive
search is great given an infinite timeout, to improve our current
coverage, we also add a randomised smoketest of partial mmaps. So let's
do both, add a randomised smoketest of partial tiling chunks and the
exhaustive (though time limited) search for failures.

Even in adding another subtest, we should shave 100s off BAT! (With,
hopefully, no loss in coverage, at least over multiple runs.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910121009.13431-1-chris@chris-wilson.co.uk