drm/arcpgu: Make ARC PGU usable on simulation platforms

In case of simulation there's no real encoder/transmitter device
because in the model's virtual LCD  we're rendering whatever
appears in frame-buffer memory.

Signed-off-by: Ruud Derwig <rderwig@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>