Changed the mapping between gl::BufferUsage flags
and D3DBufferUsage flags. Only gl::BufferUsage::DynamicDraw
gets mapped to D3DBufferUsage::DYNAMIC now. This reflects
a better mapping between GL and D3D.

BUG=angleproject:3366

Change-Id: I5062f91fdb3664339e2c259b399d5f47401675d6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1569465Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>