drm/amdgpu: Use atomic function to disable crtcs with dc enabled

This change fixes the deadlock when unloading the driver with displays
connected.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>