drm/amdgpu/vkms: fix a possible null pointer dereference

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu/vkms: fix a possible null pointer dereference In amdgpu_vkms_conn_get_modes(), the return value of drm_cvt_mode() is assigned to mode, which will lead to a NULL pointer dereference on failure of drm_cvt_mode(). Add a check to avoid null pointer dereference.