intel: Add an assert for glCopyTexSubImage() being called on MSAA buffers.

intel: Add an assert for glCopyTexSubImage() being called on MSAA buffers.

This is just in case someone else trips over this due to our weird reuse
of this code in glBlitFramebuffer().

Reviewed-and-tested-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Paul Berry <stereotype441@gmail.com>