Set default color mode for all displays

Virtual displays were not getting their color mode set
and that confused SurfaceFlinger when it was working with
virtual displays. Now set the display to the same default
color mode as the default display.
Test: b/37102733
      Should not see screen turn green when enabling
      "simulate secondary displays" function.
bug: 37102733

Change-Id: I4d8f9e8d5c191623a286b03a05841bdeb20718b1