drm/amd/display: Add clock control callbacks

[why & how]
Add clock source selection control functions based on spec

Reviewed-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Signed-off-by: Hansen Dsouza <Hansen.Dsouza@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>