DRM: remove unused dev_class

The struct class_device *dev_class is not used in the struct drm_head
structure at all, so remove it as class_device is being removed entirely
from the kernel.


Cc: David Airlie <airlied@linux.ie>
Cc: Tony Jones <tonyj@suse.de>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>