VR HWC: Relax checks around layer presentation

Rather than error out on layers without buffers, simply discard them.
This is more in line with other HWComposer implementations which deal
with this error, since SurfaceFlinger doesn't handle it appropriately.

Bug: 37091090
Test: Ran on device and verified errors are no longer reported in logcat
Change-Id: Iacfd9fa707d86367c7b6dbf62edea297fc9dfb8e