drm: Include task->name and master status in debugfs clients info

Showing who is the current master is useful for trying to decypher
errors when trying to acquire master (e.g. a race with X taking over
from plymouth). By including the process name as well as the pid
simplifies the task of grabbing enough information remotely at the point
of error.

v2: Add the command column header and flesh out a couple of comments.
(David Herrmann)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>