drm/radeon: remove invalid pci id

commit 8c3e4347 upstream.

0x4c6e is a secondary device id so should not be used
by the driver.

Noticed-by: Mark Kettenis <mark.kettenis@xs4all.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>