[PATCH] fix drm_i915_flip_t breakage

[Jesse B. <jes...@in...>]

In one of my overzealous pipe/plane mapping patches, I renamed the pipes 
field in drm_i915_flip_t to planes, since it's really dealing with 
planes and so seemed to make sense.

However, drm_i915_flip_t has different compatibility requirements than 
the i915 sarea private.  The sarea private is duplicated in the Mesa 
and DDX trees and used under a different name, so changes to it, as 
long as they're binary compatible, are fine.  drm_i915_flip_t, on the 
other hand, is used under the same name by both the Mesa and DDX trees.

So if you build an old DDX or Mesa against a new DRM, they'll pick up 
the new drm_i915_flip_t definition (since HAVE_I915_FLIP will be 
defined) and promptly break.  This patch reverts the DRM to the old 
name and adds comments describing the actual field usage.

Sorry for the trouble.

Jesse