If the uclass_find_device_by_of_offset display device probe function won't be
called. Display device's pinctrl will not take effect. SO we use
uclass_get_device_by_of_offset to get display device.
Change-Id: I1329eb5b5ffca7609dfc52bcc5623476f99bc8b0
Signed-off-by: Algea Cao <algea.cao@rock-chips.com>