i965: Terminate brw_supported_msaa_modes() list with -1, not 0

This simplifies the loop logic in a subsqequent patch that refactors
intel_quantize_num_samples() to use brw_supported_msaa_modes().

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
i965: Share code between intel_quantize_num_samples and clamp_max_samples

clamp_max_samples() and intel_quantize_num_samples() each maintained
their own list of which MSAA modes the hardware supports. This patch
removes the duplication by making intel_quantize_num_samples() use the
same list as clamp_max_samples(), the list maintained in
brw_supported_msaa_modes().

By removing the duplication, we prevent the scenario where someone
updates one list but forgets to update the other.

Move function `brw_context.c:static brw_supported_msaa_modes()` to
`intel_screen.c:(non-static) intel_supported_msaa_modes()` and patch
intel_quantize_num_samples() to use the list returned by that function.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
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. The neglect to
alter the context version is a fatal issue for some apps.

For example, consider running Chromium with clamp_max_samples=0.
Pre-patch, Mesa creates a GL 3.0 context but clamps GL_MAX_SAMPLES to
0. This violates the GL 3.0 spec, which requires GL_MAX_SAMPLES >= 4.
   The spec violation causes WebGL context creation to fail in many
scenarios because Chromium correctly assumes that a GL 3.0 context
supports at least 4 samples.

Since the driconf option was introduced largely for Chromium, the issue
really needs fixing.

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.

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