drm/amd/display: Fix psr static frames calculation
[Why]
Driver crash with psr feature enabled due to divide-by-zero error.
This is a regression after rework to calculate static screen frame
number entry time.
[How]
Correct order of operations to avoid divide-by-zero.
Signed-off-by: Roman Li <roman.li@amd.com> Reviewed-by: Zhan Liu <Zhan.Liu@amd.com> Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>