drm/amdgpu: Add amdgpu <--> amdkfd gfx8 interface

This patch adds the gfx8 interface file between amdgpu and amdkfd. This
interface file is currently in use when running on a Carrizo-based
system.

The interface itself is represented by a pointer to struct
kfd_dev. The pointer is located inside amdgpu_device structure.

All the register accesses that amdkfd need are done using this
interface. This allows us to avoid direct register accesses in
amdkfd proper, while also allows us to avoid locking between
amdkfd and amdgpu.

The single exception is the doorbells that are used in both of
the drivers. However, because they are located in separate pci
bar pages, the danger of sharing registers between the drivers
is minimal.

Having said that, we are planning to move the doorbells as well
to amdgpu.

Signed-off-by: Ben Goz <ben.goz@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>