state_tracker: Fix ReadPixels() error when mip-level > 0
Fixes assert in iris driver when ReadPixels() called to read from FBO to GL_PIXEL_PACK_BUFFER, on mip-level > 0, since num_layers wasn't properly calculated with mip-level. Fixes: 3c979b0e('iris: add some draw resolve hooks') Signed-off-by: Mykhailo Skorokhodov mykhailo.skorokhodov@globallogic.com