onscreen/native: Clear only next_fb in page_flip_feedback_discarded

When a flip is discarded it is next_fb that failed. However by calling meta_onscreen_native_swap_drm_fb we would end up clearing current_fb that the kernel is still scanning out, and keeping a reference to next_fb. That was the opposite of what we want.

What we want when a next_fb flip fails is to keep current_fb because that's what's still scanning out (probably), and to clear the reference to next_fb that's being discarded.