st/mesa: only enable MSAA coverage options when we have a MSAA buffer

st/mesa: only enable MSAA coverage options when we have a MSAA buffer

Regardless of whether GL_MULTISAMPLE is enabled (it's enabled by default)
we should not set the alpha_to_coverage or alpha_to_one flags if the
current drawing buffer does not do MSAA.

This fixes the new piglit gl-1.3-alpha_to_coverage_nop test.

ETQW is a game that enables GL_SAMPLE_ALPHA_TO_COVERAGE without MSAA.
Shrubs along the side of roads were invisible because fragments with
alpha < 0.5 were being discarded (zero coverage).

v2: remove ctx->DrawBuffer != NULL check.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>