Rename drivers/dri/i915pipe --> drivers/dri/intel_winsys.

This is appropriate as this is not a "pipe driver for the i915" as the
old name would suggest, but rather a dri driver that can provide "winsys"
backends to softpipe/i915/i965/etc pipe drivers, running under the
intel DDX, drm, etc.

It also frees up the i915pipe name for something more appropriate, ie
mesa/pipe/i915pipe.

configs/linux-dri

@@ -66,4 +66,4 @@ WINDOW_SYSTEM=dri
 
 # gamma are missing because they have not been converted to use the new
 # interface.
-DRI_DIRS = i915pipe 
+DRI_DIRS = intel_winsys