mesa/es: Allow GL_CLIP_PLANE0+6 and GL_CLIP_PLANE0+7.
mesa/es: Allow GL_CLIP_PLANE0+6 and GL_CLIP_PLANE0+7.

Fixes the ES1 conformance 'userclip' test, which broke when we increased
MAX_CLIP_PLANES to 8.  Core Mesa already validates incoming values
against MAX_CLIP_PLANES; we just need the ES wrapper to pass everything
through.

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

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