Increase MAX_CLIP_PLANES to 8

Increase MAX_CLIP_PLANES to 8

This will allow drivers to increase ctx->Const.MaxClipPlanes to 8,
which is required for GLSL-1.30 compliance.

No driver behavior should be affected.  However, many data structures
use MAX_CLIP_PLANES as an array size, so these arrays will get
slightly larger.

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