drm: fix fallouts from slow-work -> wq conversion

Commit 991ea75c (drm: use workqueue instead of slow-work), which made
drm to use wq instead of slow-work, didn't account for the return
value difference between delayed_slow_work_enqueue() and
queue_delayed_work().  The former returns 0 on success and -errno on
failures while the latter never fails and only uses the return value
to indicate whether the work was already pending or not.

This misconversion triggered spurious error messages.  Remove the now
unnecessary return value check and error message.

Markus: caught another incorrect conversion in drm_kms_helper_poll_enable()

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Tested-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org