tests/i915/gem_exec_capture: Restore engines

The test was updated some engine properties but not restoring them
afterwards. That would leave the system in a non-default state which
could potentially affect subsequent tests. Fix it by using the new
save/restore engine properties helper functions.

v2: Don't restore too soon in the 'pi' test.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>