drm/amdgpu: Fix dce_v6_0_disable_dce warning

drm/amdgpu: Fix dce_v6_0_disable_dce warning

Include a missing header to get rid of the following warning:

drivers/gpu/drm/amd/amdgpu/dce_v6_0.c:521:6: warning: no previous prototype for 'dce_v6_0_disable_dce' [-Wmissing-prototypes]
 void dce_v6_0_disable_dce(struct amdgpu_device *adev)
      ^

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>