drm/amdgpu: fix unused-function error

If CONFIG_DRM_AMDGPU=y and CONFIG_DRM_AMD_DC is not set,
gcc complained about unused-function :

drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1705:13: error: ‘amdgpu_discovery_set_sriov_display’ defined but not used [-Werror=unused-function]
 static void amdgpu_discovery_set_sriov_display(struct amdgpu_device *adev)
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

To fix this error, use CONFIG_DRM_AMD_DC to wrap
the definition of amdgpu_discovery_set_sriov_display().

Fixes: 25263da3 ("drm/amdgpu: rework SR-IOV virtual display handling")
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>