drm/rockchip: vop: Quiet always-warning AFBC log

The downstream code from which this was derived didn't ever run through
this 'switch' block with non-AFBC formats, but the upstream code does --
we use this function to probe whether a given format is supported.

Demote the warning to eliminate this sort of warning seen on every
boot:

  [drm] unsupported AFBC format[3231564e]

And make it warn more than once, because if we *actually* care to see
what formats we're probing/rejecting and for what reasons, we probably
care about more than just the first message.

Drop the comment, because one of the two *is* commonly reachable.

And lastly, drop the unreachable return; we'd do better to let the
compiler complain if we start hitting this unexpectedly.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031101557.1.Ic1569d394173c1c3016142fee4bb87a09753db94@changeid