drm-formats: avoid asserting when trying to add existing formats

If the requested to be added format is already present, return it gracefully instead of forcefully aborting the executable altogether.

Exposing duplicate formats is, in the end, a bug in the driver and must be fixed there. Since it does not prevent Weston from working properly, the issue can be logged and program execution can continue.

Signed-off-by: Francesco Giancane quic_fgiancan@quicinc.com