compositor-drm: Add Support virtual output

Add support virtual output for streaming image to remote output by
remoting-plugin which will be added by the patch:
"Add remoting plugin for output streaming."
The gbm bo of virtual output is the linear format.

Virtual output is implemented based on a patch by Grigory Kletsko
<grigory.kletsko@cogentembedded.com>.

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>