Some NVIDIA D3D11 drivers are buggy and interprets the level passed to
GetDimensions as being relative to 0, rather than the SRV's MostDetailedMip.
This affects the implementation of the imageSize function in the D3D11 backend.

Bug: angleproject:3100
Change-Id: I1e48f5df5e40caf49a4d07662aec587e98cf8388
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1677206Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>