gpu_display: clean up EventDevice buffer mgt.
gpu_display: clean up EventDevice buffer mgt.

The code to manage the EventDevice buffers was a little tricky to
follow. This CL refactors it.

BUG=b:239699447
TEST=builds. Tested downstream.

Change-Id: I979643ed82dc69544d2073d09f55a52a4cc3bb39
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3782777
Tested-by: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Commit-Queue: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm