eglnative: Simplify and remove 'fb_in_use'

Now instead of returning two booleans from meta_gpu_kms_flip_crtc with four possible meanings, we just return one (success or failure). The caller no longer needs to know if we succeeded via a flip, or succeeded via setting the CRTC.

This allows us to remove the parameter from multiple functions and to remove duplicate logic from meta_onscreen_native_flip_crtcs.