msm: kgsl: Reset ringbuffer pointers if CP INIT fails

This helps to avoid a corner case where preemption path sees the mismatch
in rptr and wptr, and misinterprets it as if a real cmdbatch was submitted,
and tries to trigger preemption on a power collapsed device, which then
leads to NoC errors.

Change-Id: I98fd98fb0b2b01fc7fd997d2dbe44ea5109d4bea
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>