mesa: set up gl_vert_result and gl_frag_attrib values for gl_ClipDistance.

mesa: set up gl_vert_result and gl_frag_attrib values for gl_ClipDistance.

This patch assigns enumerated values for gl_ClipDistance in the
gl_vert_result and gl_frag_attrib enums, so that driver back-ends can
assign gl_ClipDistance to the appropriate hardware registers.  It also
adjusts the functions _mesa_vert_result_to_frag_attrib() and
_mesa_frag_attrib_to_vert_result() (which translate between the two
enums) to correctly translate the new enumerated values.

Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Brian Paul <brianp@vmware.com>