drm/amd/display: Remove unused value set from 'min_hratio_fact' in dml

These portions of code are flagged as 'UNUSED_VALUE' by the
Coverity analysis since the assigned values of these vars
are never used in the code.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Ivan Lipski <ivlipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>