drm/msm/a6xx: Track and manage a6xx state memory

The a6xx GPU state allocates a LOT of memory. Add a bit of
infrastructure to track the memory allocations in the GPU structure
and delete them when the state is destroyed much the same way
that devm works with the device model as a whole.  This protects
against the developer accidentally forgetting to add a kfree() to
an ever growing list.
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>