drm/i915: Ditch the i915_gem_ww_ctx loop member

It's only used by the for_i915_gem_ww() macro and we can use
the (typically) on-stack _err variable in its place.

v2:
- Don't clear the _err variable when entering the loop
  (Matthew Auld, Maarten Lankhorst).
- Use parentheses around the _err macro argument.
- Fix up comment.

Cc: Matthew Auld <matthew.william.auld@gmail.com>
Suggested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
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/20210816171444.105469-1-thomas.hellstrom@linux.intel.com