vr_hwc: Address ion buffer leakage

Problem:
system is running out of memory while switching between apps back and forth.

Root-cause:
vr_hwc is leaking ion buffers by holding an unreleased reference.

Solution:
No need to register the buffer when getting buffer info. This removes the
additional (unreleased) reference on the buffer.

Test: Validated vr_hwc is no longer leaking ion buffers when switching
      apps.
Bug: 36449218

Change-Id: I1c1c5d2cca35c50a4bcbcf7b879d37914b8d835f