backend-rdp: report a zero physical size to compositor

backend-rdp: report a zero physical size to compositor

The RDP-backend is reporting a non-zero physical size
value, and there are some clients that get the resolution
in pixels directly from the physical size reported. This
leads to a resolution of 25.4 PPI (or 1px/1mm), which is too
small.

But there's no need for that. The physical size is reported
on enabling the output (in the case of RDP-backend we have
no information about it before this), and the resolution is
already set in this moment.

Report a zero physical size to compositor, what makes frontend
and clients use their default values and applications become
readable.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>