Buffers with dynamic usage will have frequent CPU updates.
Don't CPU unmap such buffers after every update. Commits
b5af8bde and 58c35d42 took care of performing an unmap when
we release the buffer either to the renderer or mBufferFreeList.

Bug: angleproject:5689
Change-Id: Ib6b8f6a7d0cb36583140e67bf164e074af098b8b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2741688Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>