i965: Set the maximum VPIndex

i965: Set the maximum VPIndex

At various stages the hardware clamps the gl_ViewportIndex to these
values.  Setting them to zero effectively makes gl_ViewportIndex be
ignored.  This is acutally useful in blorp (so that we don't have to
modify all of the viewport / scissor state).

v2: Use INTEL_MASK to create GEN6_CLIP_MAX_VP_INDEX_MASK.  Suggested by
Ken.

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