st/mesa: Go back to using ctx->Array.RestartIndex, not _RestartIndex.

st/mesa: Go back to using ctx->Array.RestartIndex, not _RestartIndex.

The derived _RestartIndex field is an attempt to support both
GL_PRIMITIVE_RESTART and GL_PRIMITIVE_RESTART_FIXED_INDEX (part of ES
3.0).  Gallium drivers don't appear to support ES 3.0 yet, so they don't
need to use it.  Plus, it's broken and going to go away soon.

NOTE: This is a candidate for the 9.1 branch.

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