drm/radeon: fix a possible null pointer dereference

In the Linux kernel, the following vulnerability has been resolved: drm/radeon: fix a possible null pointer dereference In radeon_fp_native_mode(), the return value of drm_mode_duplicate() is assigned to mode, which will lead to a NULL pointer dereference on failure of drm_mode_duplicate(). Add a check to avoid npd. The failure status of drm_cvt_mode() on the other path is checked too.