drm/amd/display: prevent potential division by zero errors

There are two places in apply_below_the_range() where it's possible for
a divide by zero error to occur. So, to fix this make sure the divisor
is non-zero before attempting the computation in both cases.

Cc: stable@vger.kernel.org
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2637
Fixes: a463b263 ("drm/amd/display: Fix frames_to_insert math")
Fixes: ded6119e

 ("drm/amd/display: Reinstate LFC optimization")
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>