drm/amd/display: do not compare integers of different widths

[Why & How]
Increase width of some variables to avoid comparing integers of
different widths

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>