Use ww locking for pin_to_display_plane for all the pinning and locking.
With the locking removed from set_cache_level, we need to fix
i915_gem_set_caching_ioctl to take the object reservation lock.

As this is a single lock, we don't need to use the ww dance.

Changes since v1:
- Do not use ww locking in i915_gem_set_caching_ioctl (Thomas).
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200819140904.1708856-24-maarten.lankhorst@linux.intel.comSigned-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>