drm/amd/display: add root clock control function pointer to fix display corruption

[Why and how]

External display has corruption because no root clock control function. Add the function pointer to fix the issue.

Reviewed-by: Daniel Miess <daniel.miess@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Xi (Alex) Liu <xi.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>