drm/msm/a6xx: Fix PDC register overlap

The current design greedily takes a big chunk of the PDC
register space instead of just the GPU specific sections
which conflicts with other drivers and generally makes
a mess of things.

Furthermore we only need to map the GPU PDC sections
just once during init so map the memory inside the function
that uses it and adjust the pointers and register offsets
accordingly.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>