dt-bindings: fsl-imx-drm: fix example compatible string

Example `display-subsystem` has an incorrect compatible string.

Required properties section tells that developers should use
"fsl,imx-display-subsystem" as "compatible" string but the example
misses 'imx-' prefix.

Change example to have correct "compatible" string.

Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
Link: https://lore.kernel.org/r/20201113150618.417424-1-cengiz@kernel.wtf


Signed-off-by: Rob Herring <robh@kernel.org>