st/mesa: fix import of EGL images with non-zero level or layer

st/mesa: fix import of EGL images with non-zero level or layer

In GL state, textures created from EGL images look like plain 2D textures
with a single level, so we use the existing layer_override facility and
add an analogous level_override one.

Fixes dEQP-EGL.functional.image.create.gles2_cubemap_{positive,negative}_{x,y,z}_rgba_texture

Reviewed-by: Marek Olšák <marek.olsak@amd.com>