drm/amd/display: Enable DCN clock gating for DCN35

[WHY & HOW]
Enable DCN clock gating for DCN35.
Disable DTBCLK gate before link training
and re-enable afterwards

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Daniel Miess <daniel.miess@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>