drm/amd/display: disable DPP RCG before DPP CLK enable

[why]
DPP CLK enable needs to disable DPPCLK RCG first.
The DPPCLK_en in dccg should always be enabled when the corresponding
pipe is enabled.

Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>