drm/amd/display: Check denominator pbn_div before used

[WHAT & HOW]
A denominator cannot be 0, and is checked before used.

This fixes 1 DIVIDE_BY_ZERO issue reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>