drm/msm: Handle register offset differences between a3xx and a4xx

Register offsets have changed between a3xx and a4xx GPUs.
To be able access these registers in common code, we create
a lookup table, and set of read-write APIs to access the
register through the lookup table.

Signed-off-by: Aravind Ganesan <aravindg@codeaurora.org>
[robclark: remove REG_ADRENO_UNDEFINED, just use zero, and minor
tweaks for latest generated headers]
Signed-off-by: Rob Clark <robdclark@gmail.com>