drm/msm: Fix race condition in the submit path

There is a race condition issue between the IRQ context trying to
trigger preemption and the user context trying to submit commands to
the GPU. The check in a5xx_flush() API only updates the wptr if the GPU is
not in preemption. In the cases where we move from PREEMPT_START to
PREEMPT_NONE there is a small window where the preempt state is still
in START but the CPU context switches to the user thread which is in
the a5xx_flush() call to update the wptr, but fails to update the wptr to
the GPU since the preempt state is not PREEMPT_NONE. This leads to a
GPU stall.

Introduce a new intermediate state PREEMPT_ABORT and
change preempt_trigger() to use gpu's current ring instead of the
ring retrieved from get_next_ring() while in this state.

Change-Id: I333e9de19824bd373901bbc8afc829de04635017
CRs-Fixed: 2081164
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>