st/mesa: fix pipe_get_transfer() call in fallback_copy_texsubimage()

st/mesa: fix pipe_get_transfer() call in fallback_copy_texsubimage()

Commit 1a339b6c71ebab6e1a64f05b2e133022d3bbcd15 caused us to take
a different path through the glCopyTexSubImage() code.  The
pipe_get_transfer() call neglected to pass the texture's level, face
and slice info.  So we were always transferring from the 0th mipmap
level even when the source renderbuffer was a non-zero mipmap level
in a texture.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=38649

NOTE: This is a candidate for the 7.10 branch.