drm/amd/amdgpu: hide #warning for missing DC config

It is annoying to have #warnings that trigger in randconfig
builds like

drivers/gpu/drm/amd/amdgpu/soc15.c:653:3: error: "Enable CONFIG_DRM_AMD_DC for display support on SOC15."
drivers/gpu/drm/amd/amdgpu/nv.c:400:3: error: "Enable CONFIG_DRM_AMD_DC for display support on navi."

Remove these and rely on the users to turn these on.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>