intel: Kill dead code in intel_miptree_copy_teximage()

intel: Kill dead code in intel_miptree_copy_teximage()

Kill the code paths taken when src_mt is null. It is never null, otherwise
there would be a segfault on line 4 of this function:
    GLuint width = src_mt->level[level].width;

(Some interleaved lines in the diff make the real diff non-obvious. All
I did was delete some code and then left-shifted what remained to correct
the indentation.)

Reviewed-by: Eric Anholt <eric@aholt.net>
Signed-off-by: Chad Versace <chad@chad-versace.us>