i965: Add driconf option clamp_max_samples (v4)

The new option clamps GL_MAX_SAMPLES to a hardware-supported MSAA mode.
If negative, then no clamping occurs.

v2: (for Paul)
  - Add option to i965 only, not to all DRI drivers.
  - Do not realy on int->uint cast to convert negative
    values to large positive values. Explicitly check for
    clamp_max_samples < 0.
v3: (for Ken)
   - Don't allow clamp_max_samples to alter context version.
   - Use clearer for-loop and correct comment.
   - Rename variables.
v4: (for Ken)
   - Merge identical if-branches.

Reviewed-and-tested-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>