iris: fix layer calculation for TEXTURE_3D ReadPixels() on mip-level>0
Fixes assert 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.
The logic of fix is similar to one used in 'brw_predraw_resolve_inputs' function of i965 driver, which is called in a same circumstances.
Fixes: 3c979b0e ('iris: add some draw resolve hooks')
Signed-off-by: Yevhenii Kharchenko yevhenii.kharchenko@globallogic.com