drm/amdgpu: add amdgpu <--> amdkfd gfx7 interface

This patch adds the gfx7 interface file between amdgpu and amdkfd. This
interface file mirrors (some) of the functions in radeon_kfd.c
(the interface file between radeon and amdkfd).

The gfx7 interface is used when it is run on a Kaveri-based system.

This interface file was used for bring-up of amdkfd on amdgpu and for
debugging purposes. For users who would like to run HSA on Kaveri, please
use the radeon graphic driver.

Note: CONFIG_DRM_AMDGPU_CIK must be selected for amdgpu to handle Kaveri.

v2: removed MTYPE_NONCACHED enum definition as it is defined in another
patch

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