st/mesa: Allow zero as [level|layer]_override
This fix two dEQP tests for virgl:
dEQP-EGL.functional.image.create.gles2_cubemap_positive_x_rgba_texture dEQP-EGL.functional.image.render_multiple_contexts.gles2_cubemap_positive_x_rgba8_texture
Signed-off-by: Lepton Wu lepton@chromium.org