radeon_display: Use pointer return error codes

commit b2f4b03f8a378cd626d2ea67d19e7470c050a098 upstream.

drm_mode_addfb() expects fb_create return error code
instead of NULL.

Signed-off-by: xueminsu <xuemin.su@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>