drm/amd/display: Wrong index type for pipe iterator

[Why and How]
Type mismatch in index and pipe count might cause an infinite loop. code
Change should resolve this issue.

Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Saaem Rizvi <syedsaaem.rizvi@amd.com>
Reviewed-by: Josip Pavic <Josip.Pavic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>