drm/i915/selftests: Return to kernel context after each test

As we flush each test and wait for idle before the next, also switch
back to the kernel context. This helps limit the amount of collateral
damage a test may cause by resetting to the default state each time (and
also helps clean up temporaries used by the test).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508115312.12628-1-chris@chris-wilson.co.uk