lib/igt_pm: Add a helper to query the runtime_usage.

It will be useful so the tests can assert the expected
usage counter movement depending on the operation.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>