imx-drm: imx-drm-core: avoid going the long route round for drm_device

We have the drm_device available, so rather than storing it and then
using the stored version, us the one we already have available to us.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>