drm/radeon: fix null pointer dereference in radeon_add_common_modes

In radeon_add_common_modes(), the return value of drm_cvt_mode() is
assigned to mode, which will lead to a possible NULL pointer dereference
on failure of drm_cvt_mode(). Add a check to avoid npd.

Cc: stable@vger.kernel.org
Fixes: d50ba256 ("drm/kms: start adding command line interface using fb.")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>