msm: kgsl: Use a standard function for register access

There were dual functions for reading and writing registers for
adreno devices. Stop the use of one of these dual functions as
it makes the code more uniform.

Change-Id: I703d27d1674a85a6c2d7a9fe6dc49f13005a3410
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>