drm/amd/display: fix a divide by zero error

[Why&How]

Incorrect variable was being checked for zero condition.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>