i965: Set up clip distance VUE slots appropriately for gl_ClipDistance.

i965: Set up clip distance VUE slots appropriately for gl_ClipDistance.

When gl_ClipDistance is in use, the contents of the gl_ClipDistance
array just need to be copied directly into the clip distance VUE
slots, so we re-use the code that copies all other generic VUE slots
(this has been extracted to its own method).  When gl_ClipDistance is
not in use, the vertex shader needs to calculate the clip distances
based on user-specified clipping planes.

This patch also removes the i965-specific enum values
BRW_VERT_RESULT_CLIP[01], since we now have generic Mesa enums that
serve the same purpose (VERT_RESULT_CLIP_DIST[01]).

Reviewed-by: Eric Anholt <eric@anholt.net>