drm/i915: Fixup kerneldoc for struct intel_context

   commit ae6c4806
   Author: Daniel Vetter <daniel.vetter@ffwll.ch>
   Date:   Wed Aug 6 15:04:53 2014 +0200

       drm/i915: Only track real ppgtt for a context

Changed the code but didn't update kerneldoc.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: "Thierry, Michel" <michel.thierry@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -774,7 +774,7 @@ struct i915_ctx_hang_stats {
  *	       context).
  * @hang_stats: information about the role of this context in possible GPU
  *		hangs.
- * @vm: virtual memory space used by this context.
+ * @ppgtt: virtual memory space used by this context.
  * @legacy_hw_ctx: render context backing object and whether it is correctly
  *                initialized (legacy ring submission mechanism only).
  * @link: link in the global list of contexts.