imx-drm: remove separate imx-fbdev

Now that we know when the components of the imx-drm subsystem will be
initialised, we can move the fbdev helper initialisation and teardown
into imx-drm-core.  This gives us the required ordering that DRM wants
in both driver load and unload methods.

We can also stop exporting the imx_drm_device_get() and
imx_drm_device_put() methods; nothing but the fbdev helper was making
use of these.

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>