Fix emulator vsync

The code that reserves display IDs was only run when a hardware
composer was present.  The eventControl() function, which handles
enabling of vsync, was ignoring the request because the primary
display didn't appear in its set of allocated IDs.  This moves
reservation of IDs for built-in displays outside the HWC-only block.

Also, added a couple of warnings in eventControl().

Bug 7376568

Change-Id: I185ccdf817a25499b5c2668f8f6d594afb8c1568