[dawn][d3d12] Fix compressed texture clear staging buffer size.

The computations for `uploadSize` didn't stay in block space until the
end, and instead included two factors needed to convert from Block space
to bytes.

Fix this by using the exact same logic as in TextureVk that does the
conversion from blocks to bytes only once, at the end.

Also rework the same code in TextureMTL to stay in block space as much
as possible.

Fixed: 476798059
Change-Id: I0ea92f02701db591a356a4007abe9ede4894e2c2
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/286575
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>