drm/i915/selftests: Calculate maximum ring size for preemption chain

32 is too many for the likes of kbl, and in order to insert that many
requests into the ring requires us to declare the first few hung --
understandably a slow and unexpected process. Instead, measure the size
of a singe requests and use that to estimate the upper bound on the
chain length we can use for our test, remembering to flush the previous
chain between tests for safety.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: "Yokoyama, Caz" <caz.yokoyama@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321194031.20240-1-chris@chris-wilson.co.uk