mesa: Add validation helpers for new indirect draws

mesa: Add validation helpers for new indirect draws

Based on part of Patch 2 of Christoph Bumiller's ARB_draw_indirect series.

V3: - Disallow primcount==0 for DrawMulti*Indirect. The spec is unclear
      on this, but it's silly. We might go back on this later if it
      turns out to be a problem.

    - Make it clear that the caller has dealt with stride==0

V4: - Allow primcount==0 again.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>