st/mesa: fix front buffer regression after dropping st_validate_state in Blit

Broken by: d082c53249
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92072

Cc: 10.6 11.0 <mesa-stable@lists.freedesktop.org>
Tested-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/mesa/state_tracker/st_cb_blit.c

@@ -229,6 +229,7 @@ st_BlitFramebuffer(struct gl_context *ctx,
                   st_adjust_blit_for_msaa_resolve(&blit);
 
                   st->pipe->blit(st->pipe, &blit);
+                  dstRb->defined = true; /* front buffer tracking */
                }
             }
          }
@@ -266,6 +267,7 @@ st_BlitFramebuffer(struct gl_context *ctx,
                   st_adjust_blit_for_msaa_resolve(&blit);
 
                   st->pipe->blit(st->pipe, &blit);
+                  dstRb->defined = true; /* front buffer tracking */
                }
             }
          }