drm/i915/selftests: Only switch to kernel context when locked

In igt_flush_test() we try to switch back to the kernel context, but we
are only able to do so when we are called with struct_mutex held.

More of my CI fallout from lockdep being temporarily suppressed :(

Fixes: 4cdf65ce ("drm/i915/selftests: Return to kernel context after each 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/20180509065926.19207-1-chris@chris-wilson.co.uk