drm/amd/display: fix crash on setmode when mode is close to bw limit

[why]
during It's possible to call dcn_validate_bandwidth with no plane.
In that case, as we are only intersted in if output timing is supported or not,
even if we cannot support native resolution, we still want to support lower
resolution if it is valid

[how]
if there exist no surface, make viewport/rec_out size at max 1080p. It is
already known that 1080p x 6(max # of pipes) is supported, so if we fail
validation, it is because of the stream

Signed-off-by: Su Sung Chung <Su.Chung@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>