drm/amd/display: wait for fence without holding reservation lock

Don't block others while waiting for the fences to finish, concurrent
submission is perfectly valid in this case and holding the lock can
prevent killed applications from terminating.

Signed-off-by: Christian König <ckoenig.leichtzumerken@gmail.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>