Hardcode the default value of ctx->Const.MaxClipPlanes to 6.

Hardcode the default value of ctx->Const.MaxClipPlanes to 6.

Previously this value was set to MAX_CLIP_PLANES, which is defined to
be 6.  But MAX_CLIP_PLANES needs to be increased to 8 to support
GLSL-1.30-compliant drivers.  This patch hard-codes the default value
of ctx->Const.MaxClipPlanes to 6, so that when MAX_CLIP_PLANES is
increased, it won't affect drivers that do not support 8 clip planes.

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