Add a shadow pointer to track the current command being written into
the ring. Don't commit it as 'cur' until the command is submitted.
Because 'cur' is used to construct the software copy of the wptr this
ensures that somebody peeking in on the ring doesn't assume that a
command is inflight while it is being written. This isn't a huge deal
with a single ring (though technically the hangcheck could assume
the system is prematurely busy when it isn't) but it will be rather
important for preemption where the decision to preempt is based
on a non-empty ringbuffer. Without a shadow an aggressive preemption
scheme could assume that the ringbuffer is non empty and switch to it
before the CPU is done writing the command and boom.

Even though preemption won't be supported for all targets because of
the way the code is organized it is simpler to make this generic for
all targets. The extra load for non-preemption targets should be
minimal.
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>