i965: Remove spurious mutex frobbing around call to intel_miptree_blit

i965: Remove spurious mutex frobbing around call to intel_miptree_blit

These locks were added in 2f28a0dc, but I don't see anything in the
intel_miptree_blit path that should make this necessary.

When asked, Kristian says:

    I doubt it's needed now with the new blorp. If I remember correctly,
    I had to drop the lock there since intel_miptree_blit() could hit
    the XY blit path that requires a fast clear resolve. The fast
    resolve being meta, would then try to lock the texture again.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>