drm/amd/display: Fix divide-by-zero in DPPCLK and DISPCLK calculation

[Why]
Certain use cases will pass in zero in the new_clocks parameter for all
clocks. This results in a divide-by-zero error when attempting to round
up the new clock.

When new_clocks are zero, no rounding is required, so we can skip it.

[How]
Guard the division calculation with a check to make sure clocks are not
zero.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>