Revert "imx: make use of loader_open_render_node(..) helper"

Revert "imx: make use of loader_open_render_node(..) helper"

This reverts commit 773d6ea6e715d207bda3a53a9dfc8acf686035b0.

Since kernel 4.17 (drm/etnaviv: remove the need for a gpu-subsystem DT
node) the etnaviv DRM driver doesn't have an associated DT node
anymore. This is technically correct, as the etnaviv device is a
virtual device driving multiple hardware devices.

Before 4.17 the userspace had access to the following information:
DRIVER=etnaviv
OF_NAME=gpu-subsystem
OF_FULLNAME=/gpu-subsystem
OF_COMPATIBLE_0=fsl,imx-gpu-subsystem
OF_COMPATIBLE_N=1
MODALIAS=of:Ngpu-subsystemT<NULL>Cfsl,imx-gpu-subsystem
DRIVER=imx-drm
OF_NAME=display-subsystem
OF_FULLNAME=/display-subsystem
OF_COMPATIBLE_0=fsl,imx-display-subsystem
OF_COMPATIBLE_N=1

Afer 4.17:
DRIVER=etnaviv
MODALIAS=platform:etnaviv

The OF node has never been part of the etnaviv UABI, simply due to the
fact that it's still possible to instantiate the etnaviv driver from a
platform file, instead of a devicetree node.

A patch set to fix this problem was send out [1] but it looks like
that a proper solution needs more time to bake.

[1] https://lists.freedesktop.org/archives/dri-devel/2018-October/194651.html

Suggested-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>