drm: Use dev->name as fallback for dev->unique

Lots of arm drivers get this wrong and for most arm boards this is the
right thing actually. And anyway with most loaders you want to chase
sysfs links anyway to figure out which dri device you want.

This will fix dmesg noise for rockchip and sti.

Also add a fallback to driver->name for entirely virtual drivers like
vgem.

v2: Rebase on top of

commit e112e593
Author: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Date:   Fri Dec 11 11:20:28 2015 +0100

    drm: use dev_name as default unique name in drm_dev_alloc()

and simplify a bit. Plus add a comment.

v3: WARN_ON(!dev->unique) as discussed with Emil.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Reported-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466499262-18717-3-git-send-email-daniel.vetter@ffwll.ch