prime_self_import: Assure no pending requests before object counting

prime_self_import: Assure no pending requests before object counting

We don't want a previously used object to be freed in the middle of a
before/after object counting operation (or we would get a "-1 objects
leaked" message). We have seen this happening, e.g., when a context
from a previous run dies, but its backing object is alive waiting for
a retire_work to kick in.

v2: Use igt_debugfs facilities for drop cache.
v3: Move igt_drop_caches_set() call inside get_object_count() to make
it clearer why we want this.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>