drm/amdgpu: Add H/W agnostic amdgpu <--> amdkfd interface

This patch adds an interface file between amdgpu and amdkfd. This
interface file is H/W agnostic, thus containing functions that
operate the same for any AMD APU/GPU H/W generation.

The functions in this interface mirror (some) of the functions in
radeon_kfd.c (the radeon<-->amdkfd interface file). The main functions
are:

- amdgpu_amdkfd_init - initialize the amdkfd module
- amdgpu_amdkfd_load_interface - load the H/W interface according to the
  currently probed device
- amdgpu_amdkfd_device_probe - probe the device in amdkfd
- amdgpu_amdkfd_device_init - initialize the device in amdkfd
- amdgpu_amdkfd_interrupt - call the ISR of amdkfd
- amdgpu_amdkfd_suspend - suspend callback from amdgpu
- amdgpu_amdkfd_resume - resume callback from amdgpu

This patch also modifies the relevant amdgpu files, to use this new
interface.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>