The commit b5af8bde moved the unmap call on a buffer to
when it is placed in mBufferFreeList. There was one place
that was missed, rectifying that oversight here.

Bug: angleproject:5689
Change-Id: Iaed2916de374d55ab59276e2679c23ed7ef9c4e8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2733476
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>