iris: Don't skip fast depth clears if the color changed
We depend on BLORP to convert the clear color and write it into the clear color buffer for us. However, we weren't bothering to call blorp in the case where the state is ISL_AUX_STATE_CLEAR. This leads to the clear color not getting properly updated if we have back-to-back clears with different clear colors. Technically, we could go out of our way to set the clear color directly from iris in this case but this is a case we're unlikely to see in the wild so let's not bother. This matches what we already do for color surfaces.