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: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Brian Chang <Brian.Chang@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>