Remove host cached requirement for mappable buffers

Using mappable buffers required a memory type that supports both host
cached and host coherent memory. No memory of this type is available on
Mali GPUs so mappable buffers weren't used.

Drop the host cached requirement since it's not strictly necessary. This
has similar performance to dropping the host coherent requirement,
except that requires adding explicit flush/invalidate memory range calls
which is a bit more complicated. Likely dawn should pick host cached
or host coherent based on some heuristic in the future.

Bug: 422798184
Change-Id: I3f2182ee362a732c130c7946b6f31709bc02a55d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/247895
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Kyle Charbonneau <kylechar@google.com>