drm/amd/display: Blank for uclk OC in dm instead of dc

[Why]
All displays need to be blanked during the uclk OC interface so that we can
guarantee pstate switching support. If the display config doesn't support
pstate switching, only using core_link_disable_stream will not enable it
as the front-end is untouched. We need to go through the full plane removal
sequence to properly program the pipe to allow pstate switching.

[How]
 - guard clk_mgr functions with non-NULL checks

Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>