msm: kgsl: Dump the RB registers correctly on A6XX

The sub-block select register needs to be programmed to access the
RB registers. Program it before dumping these registers, both over
AHB, as well as crashdump.

Change-Id: I884737aa51571f984a8ca12f6dd390746d70bcd9
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>