i965: Fix context initialization after 2f896627175384fd5

You can't return stack-initialized values and expect anything good to
happen.
i965: Let driconf clamp_max_samples affect context version

Commit 2f89662 added the driconf option 'clamp_max_samples'.  In that
commit, the option did not alter the context version.

Specifically, Mesa constructed a GL 3.0 context for Baytrail even if
clamp_max_samples=0, which clamps GL_MAX_SAMPLES to 0. This violates the
GL 3.0 spec, which requires GL_MAX_SAMPLES >= 4.

The spec violation causes WebGL fail on Chromium because it correctly
assumes that a GL 3.0 context supports at least 4 samples.

This patch fixes calculation of the context version to respect the
post-clamped value of GL_MAX_SAMPLES. This in turn fixes WebGL on
Chromium when clamp_max_samples=0.

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