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.