All BufferVk methods need to honor the abstraction provided by
mapRangeImpl and unmapImpl. Do not map BufferVk::mBuffer directly,
this is needed for when we support device local buffers that
cannot be CPU mapped.

Bug: angleproject:5909
Change-Id: I520e5cc0994560a3784b8978e349550211dc2cde
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2862559Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>