drm/amd/display: Remove ASSERT if significance is zero in math_ceil2

In the DML math_ceil2 function, there is one ASSERT if the significance
is equal to zero. However, significance might be equal to zero
sometimes, and this is not an issue for a ceil function, but the current
ASSERT will trigger warnings in those cases. This commit removes the
ASSERT if the significance is equal to zero to avoid unnecessary noise.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 332315885d3ccc6d8fe99700f3c2e4c24aa65ab7)