mesa: Update clipping code to use ctx->Const.MaxClipPlanes.

mesa: Update clipping code to use ctx->Const.MaxClipPlanes.

To support GLSL 1.30, we will need to increase MAX_CLIP_PLANES to 8.
To avoid breaking drivers that do not yet support 8 clip planes, this
patch modifies the Mesa core code that pertains to clipping to use
ctx->Const.MaxClipPlanes rather than MAX_CLIP_PLANES, since
ctx->Const.MaxClipPlanes will remain 6 for drivers that only support 6
clip planes.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>