drm/amd/display: Check for dmcu initialization before calling dmcu

[Why]
DMCU firmware is not required - the system is expected to run
fine without it.  Therefore, wherever dmcu functions could be
called, dmcu initialization shoudl be checked

[How]
Use the helpful hook dmcu_funcs->is_dmcu_initialized

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>