drm/i915: Take reservation lock around i915_vma_pin.

We previously complained when ww == NULL.

This function is now only used in selftests to pin an object,
and ww locking is now fixed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
[danvet: Resolve conflict because we don't have a set-domain refactor,
see
https://lore.kernel.org/intel-gfx/20210203090205.25818-8-chris@chris-wilson.co.uk/



The really worrying thing here is that the above patch had a change in
arguments for i915_gem_object_set_to_gtt_domain(), without any
explanation. I decided to just faithfully apply Maarten's change but
not the argument change which was in Maarten's context diff.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-26-maarten.lankhorst@linux.intel.com