drm/amd/display: Guard against invalid array access

[Why]
There are scenarios where no OPP is assigned to an OTG so its value is
0xF which is outside the size of the OPP array causing a potential
driver crash.

[How]
Change the assert to an early return to guard against access.  If
there's no OPP assigned already, then OTG will be blank anyways so no
functionality should be lost.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>