drm/amd/display: Cast int to float before division

[Why]:
Some inputs to dml_ceil have it dividied by int which causes a
truncation. This loss of precision means the ceil function becomes
redundant and does not round up.

[How]:
Cast parameter to float before division.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>