st/mesa: Disable blending for integer formats.

Blending isn't valid for integer formats.  Rather than having drivers
worry about this, just disable blending in this case.  This hopefully
will increase hits in the CSO cache as well, by eliminating most of the
meaningless fields in this case.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_atom_blend.c

@@ -171,6 +171,7 @@ st_update_blend( struct st_context *st )
       /* blending enabled */
       for (i = 0, j = 0; i < num_state; i++) {
          if (!(ctx->Color.BlendEnabled & (1 << i)) ||
+             (ctx->DrawBuffer->_IntegerBuffers & (1 << i)) ||
              !blend->rt[i].colormask)
             continue;