drm/msm: Re-trigger preemption upon command completion

Trigger preemption from the interrupt handler. This allows us
to aggressively change ringbuffers especially to lower priority
ones and finish working on pending commands.

Change-Id: Ic05213f3d02b1bb7400461edd0d19e38d5b01ec2
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>