drm/i915: Introduce a ww transaction helper

Introduce a for_i915_gem_ww(){} utility to help make the code
around a ww transaction more readable.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617063018.92802-4-thomas.hellstrom@linux.intel.com