drm/amd/display: guard DPPHY_Internal_ctrl

[why]
this register not exist in some asic, based on request remove this from
dc.

[how]
add guard for sanization.

Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>