drm/atomic: Allow userspace to use explicit sync with atomic async flips

Allow userspace to use explicit synchronization with atomic async flips.
That means that the flip will wait for some hardware fence, and then
will flip as soon as possible (async) in regard of the vblank.

Fixes: 0e26cc72 ("drm: Refuse to async flip with atomic prop changes")
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20240702212215.109696-1-andrealmeid@igalia.com