anv: Don't assume depth/stencil attachments have depth
If a secondary command buffer is used and the client provides a framebuffer and that framebuffer has a stencil-only attchment, we would try to get the aux usage for the depth component of that attachment and crash. Check the aspects of the image before looking at aux usage.
Fixes: 0d8b9c52 ("anv: Allow PMA optimization to be enabled in secondary command buffers")