drm/i915: Add ww context to intel_dpt_pin, v2.

Ensure i915_vma_pin_iomap and vma_unpin are done with dpt->obj lock held.

I don't think there's much of a point in merging intel_dpt_pin() with
intel_pin_fb_obj_dpt(), they touch different objects.

Changes since v1:
- Fix using the wrong pointer to retrieve error code (Julia)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929085950.3063191-1-maarten.lankhorst@linux.intel.com