drm/i915/selftests: try to rein in alloc_smoke

Depending on RNG we might try to fill an 8G region for every possible
order, using the smallest possible chunk size of 4K, which seems to be
very slow. Try to remedy the situation by adding an overall timeout for
the test, while also selecting each order level in a random fashion.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1310


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305204711.217783-2-matthew.auld@intel.com