drm/amd/display: Add missing instance for clock source register

[Description]
Need to add inst 5 for clk_src_regs because
there are 5 PHY instances in DCN32 & DCN321.

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>