surfaceflinger: skip buffer cache when BYPASS_IHWC

When SF talks to hwcomposer2 directly, we should skip HWC buffer
cache.  The cache works somehow similar to
IGraphicBufferConsumer::acquireBuffer, where buffer is set to
nullptr when it is already seen previously.  That confuses
hwcomposer2.

Bug: 35325794
Test: manual
Change-Id: I5306ecc9e7290bc6734091a9884024bf38667628