drm/pl111: Replace drm_dev_unref with drm_dev_put

This patch unifies the naming of DRM functions for reference counting
of struct drm_device. The resulting code is more aligned with the rest
of the Linux kernel interfaces.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180717083657.16262-1-tzimmermann@suse.de