i965: Remove early release of DRI2 miptree

i965: Remove early release of DRI2 miptree

intel_update_winsys_renderbuffer_miptree() will release the existing
miptree when wrapping a new DRI2 buffer, so we can remove the early
release and so prevent a NULL mt dereference should importing the new
DRI2 name fail for any reason. (Reusing the old DRI2 name will result
in the rendering going astray, to a stale buffer, and not shown on the
screen, but it allows us to issue a warning and not crash much later in
innocent code.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86281
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>