Change copy.BufferStream unit

Change copy.BufferStream unit to mpixel_sec to align with glbench
metrics.

BUG=b:291637728
TEST=Run it.

Change-Id: Iab6b281c2dfdbd9f0813e3d4b554025118974ba6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vkbench/+/4834793
Auto-Submit: Po-Hsien Wang <pwang@chromium.org>
Commit-Queue: Ilja Friedel <ihf@chromium.org>
Reviewed-by: Prahlad Kilambi <prahladk@google.com>
Tested-by: Ilja Friedel <ihf@chromium.org>
Tested-by: Po-Hsien Wang <pwang@chromium.org>
3 files changed