amd-gpu: remove aperture concept from the memory manager

Now everything is one big memarena instead of a separate PHYS and EMEM,
and instead of having to micromanage the split between Z160 (always PHYS
here) and GPU (99% of it's allocations come out of EMEM), we give blocks
from the same memory pool.

This should effect some more efficient allocation strategies and allow
Xorg acceleration to proceed with much less movement of data (when it
evicts for low memory in PHYS, it moves it to userspace and uses pixman
to render everything, which is CPU intensive) at the potential cost of
leaving slightly less memory around for the 3D GPU, but userspace apps
using the GPU should be doing better with texture management so as not
to soak up >32MiB of reserved GPU memory needlessly.

Moving away from gsl_memmgr to genalloc is not desirable right now since
the driver requires multiple allocation alignment orders and genalloc can
only really guarantee a minimum alignment (i.e. size of allocation
represented by each bit in the allocation bitmap).

Moving to a dma_alloc_coherent/writecombine-only model also poses a
problem since significant amounts of code expects to be able to find the
"base" GPU memory address and with individual allocations instead of a
pooled area, there is no such thing.

Signed-off-by: Matt Sealey <matt@genesi-usa.com>