i965: Respect driconf option clamp_max_samples

Clamp gen7 GL_MAX_SAMPLES to 0, 4, or 8.
Clamp gen6 GL_MAX_SAMPLES to 0 or 4.
Clamp other gens to 0.

CC: Eric Anholt <eric@anholt.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>