drm/i915: Add igt_spinner_pin() to allow for ww locking around spinner.

By default, we assume that it's called inside igt_create_request
to keep existing selftests working, but allow for manual pinning
when passing a ww context.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-34-maarten.lankhorst@linux.intel.com