drm/amd/display: fail instead of div by zero/bugcheck

[why]
If pbn_per_slot is 0, fail instead of dividing by zero and
bugchecking.

[how]
Check for zero divisor before division operation.

Signed-off-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Reviewed-by: Wyatt Wood <Wyatt.Wood@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>