drm/amdgpu: fix a possible null pointer dereference

In amdgpu_connector_add_common_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 npd.

Cc: stable@vger.kernel.org
Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>