Two issues here:
- Logic was inverted
- getTopLevel() isn't quite right. The condition in the spec is
  whether there are any nonzero levels defined; it does not require them to
  be contiguous with the base level.

Test: dEQP-EGL.functional.image.api.create_image_gles2_tex2d_rgba_level0_only
Bug: b/141916742
Change-Id: I336d628c9508bfe61d1471431cc6170454a24d1b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36888Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>