msm: kgsl: Move A3XX specific ringbuffer init out of common init

CP_QUEUE_THRESHOLDS is only used in A3XX. Move the register setting
out of common ringbuffer initialization and into A3XX specific region.

CRs-Fixed: 971153
Change-Id: I05ef504a802534f1582e62085c5b12b20ac57209
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>