msm: kgsl: Move preemption start to ringbuffer start

We should not be doing anything specific to preemption like
accessing preemption buffers in a6xx_start as a6xx_start is
called for both eCP and non eCP paths.

Change-Id: Ia7f9e7a54bcbbb6365ddd1f621e1c582faabd962
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>