drm/arcpgu: rework encoder search

Instead of using non-standard "encoder-slave" property to find
encoder let's find it by associated endpoint.

While I'm on it add corresponding log message if we don't find
any encoder and we assume that we use virtual LCD on the
simulation platform.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>