l sync interrupt handling
is no longer predictable (due to being interrupt-sourced from two
different heads running at different speeds).  This raises the
probability of the tasklet trying to run while the userspace DRI is
doing things to the GPU (and manipulating the DRM lock).

The fix is to change the relevant spinlock semantics to be the
interrupt-blocking form.  After this change I am no longer able to
trigger the lockup; the longest test run so far was 20 hours (test
stopped after that point).

Note: I have examined the places where this spinlock is being
employed; all are reasonably short bounded sequences and should be
suitable for interrupts being blocked without impacting overall kernel
interrupt response latency.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
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