msm: kgsl: Read 64 bit GPU registers where applicable

RB_BASE and IB1_BASE GPU addresses are 64 bit values on 5XX targets.
Get into the habit of reading and treating the values as 64 bit
values so we can easily transition without hiccups.

Change-Id: Ic0dedbad0b12eedecf2dd9dfc412bfa181426c48
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>