msm: kgsl: don't store rptr in struct adreno_ringbuffer

Since the rptr is written by the GPU, there's no point
in keeping a copy in the ringbuffer struct where it will
likely be out of date. If you need to look at the ringbuffer,
read it into a local variable with adreno_get_rptr().

Change-Id: Ibf1ba0b9c71a93f65a5c85a58328b2202a27af3f
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>