i965: Use prepare_external instead of make_shareable in setTexBuffer2

The setTexBuffer2 hook from GLX is used to implement glxBindTexImageEXT
which has tighter restrictions than just "it's shared".  In particular,
it says that any rendering to the image while it is bound causes the
contents to become undefined.  This means that we can do whatever aux
tracking we want between glxBindTexImageEXT and glxReleaseTexImageEXT so
long as we always transition from external in Bind and to external in
Release.

The fact that we were using make_shareable before was a problem because
it would resolve away 100% of the aux data and then throw away our
reference to the aux buffer.  If the aux data was shared with some other
application (i.e. if we're using I915_FORMAT_MOD_Y_TILED_CCS) then we
would forget that the aux data even existed for the rest of eternity.
This is fine for the first frame but any subsequent calls to
glxBindTexImageEXT would bind the texture as if it has no aux
whatsoever and no resolves would happen and texturing would happen as if
there is no aux.  This was causing rendering corruption in mutter when
running on top of X11 with modifiers.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>

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