drm/pl111: fix possible object reference leak

drm/pl111: fix possible object reference leak

The call to of_find_matching_node_and_match returns a node pointer with
refcount incremented thus it must be explicitly decremented after the
last usage.

Detected by coccinelle with the following warnings:
drivers/gpu/drm/pl111/pl111_versatile.c:333:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 317, but without a corresponding object release within this function.
drivers/gpu/drm/pl111/pl111_versatile.c:340:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 317, but without a corresponding object release within this function.
drivers/gpu/drm/pl111/pl111_versatile.c:346:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 317, but without a corresponding object release within this function.
drivers/gpu/drm/pl111/pl111_versatile.c:354:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 317, but without a corresponding object release within this function.
drivers/gpu/drm/pl111/pl111_versatile.c:395:3-9: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 317, but without a corresponding object release within this function.
drivers/gpu/drm/pl111/pl111_versatile.c:402:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 317, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Eric Anholt <eric@anholt.net> (supporter:DRM DRIVER FOR ARM PL111 CLCD)
Cc: David Airlie <airlied@linux.ie> (maintainer:DRM DRIVERS)
Cc: Daniel Vetter <daniel@ffwll.ch> (maintainer:DRM DRIVERS)
Cc: dri-devel@lists.freedesktop.org (open list:DRM DRIVERS)
Cc: linux-kernel@vger.kernel.org (open list)
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1554307455-40361-6-git-send-email-wen.yang99@zte.com.cn