screencap: Allow virtual display ID in -d switch

Ibe3679d810602456205e157fe2cb8cc510062215 restricted the argument to
PhysicalDisplayId, which prevents capture of virtual displays.

Fixes: 280785757
Test: screencap -d <id>
Change-Id: I41a8d1adaa51c6804c03f1b3084ac04ff6b4f1fa
1 file changed