imx-drm: imx-drm-core: move allocation of imxdrm device to driver load function

It is now no longer necessary to keep this structure around; we can
allocate it upon DRM driver load and destroy it thereafter without
affecting the other components now.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>