arm64: dts: imx8mn-venice-gw7902: disable gpu

Since commit 99aa29932271 ("arm64: dts: imx8mn: Enable GPU")
imx8mn-venice-gw7902 will hang during kernel init because it uses
a MIMX8MN5CVTI which does not have a GPU.

Disable pgc_gpumix to work around this. We also disable the GPU devices
that depend on the gpumix power domain and pgc_gpu to avoid them staying
in a probe deferred state forever.

Cc: Adam Ford <aford173@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Fixes: 99aa29932271 ("arm64: dts: imx8mn: Enable GPU")
Reviewed-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>