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>

src/mesa/state_tracker/st_atom_blend.c

@@ -265,9 +265,11 @@ update_blend( struct st_context *st )
 
    blend->dither = ctx->Color.DitherFlag;
 
-   if (ctx->Multisample.Enabled) {
-      /* unlike in gallium/d3d10 these operations are only performed
-         if msaa is enabled */
+   if (ctx->Multisample.Enabled &&
+       ctx->DrawBuffer->Visual.sampleBuffers > 0) {
+      /* Unlike in gallium/d3d10 these operations are only performed
+       * if both msaa is enabled and we have a multisample buffer.
+       */
       blend->alpha_to_coverage = ctx->Multisample.SampleAlphaToCoverage;
       blend->alpha_to_one = ctx->Multisample.SampleAlphaToOne;
    }

src/mesa/state_tracker/st_context.c

@@ -166,7 +166,8 @@ void st_invalidate_state(struct gl_context * ctx, GLbitfield new_state)
    struct st_context *st = st_context(ctx);
 
    if (new_state & _NEW_BUFFERS) {
-      st->dirty |= ST_NEW_DSA |
+      st->dirty |= ST_NEW_BLEND |
+                   ST_NEW_DSA |
                    ST_NEW_FB_STATE |
                    ST_NEW_SAMPLE_MASK |
                    ST_NEW_SAMPLE_SHADING |