drm: imx: Move imx-drm driver out of staging

The imx-drm driver was put into staging mostly for the following reasons,
all of which have been addressed or superseded:
 - convert the irq driver to use linear irq domains
 - work out the device tree bindings, this lead to the common of_graph
   bindings being used
 - factor out common helper functions, this mostly resulted in the
   component framework and drm of_graph helpers.

Before adding new fixes, and certainly before adding new features,
move it into its proper place below drivers/gpu/drm.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>