r300-gallium: Enable all four colorbuffer render targets.

As far as I can tell all the state emission necessary has been set up.
Well, except for the fragment shader, but c'mon, gimme a break. :3

src/gallium/drivers/r300/r300_screen.c

@@ -100,8 +100,7 @@ static int r300_get_param(struct pipe_screen* pscreen, int param)
             /* IN THEORY */
             return 0;
         case PIPE_CAP_MAX_RENDER_TARGETS:
-            /* XXX 4 eventually */
-            return 1;
+            return 4;
         case PIPE_CAP_OCCLUSION_QUERY:
             /* IN THEORY */
             return 0;