drm_hwcomposer: do not queue layers map for virtual display

It is invalid to add a DrmCompositionDisplayLayersMap for the virtual
display because the corresponding physical display resources (such as
crtc, connector, etc.) do not exist.

BUG=24985282
TEST=verify that no crash occurs

Change-Id: I24a6671e7f94032a38104ae7ed3eb0da79479601
1 file changed