drm/amdgpu: replace per_device_list by array

Remove per_device_list from kfd_process and replace it with a
kfd_process_device pointers array of MAX_GPU_INSTANCES size. This helps
to manage the kfd_process_devices binded to a specific kfd_process.
Also, functions used by kfd_chardev to iterate over the list were
removed, since they are not valid anymore. Instead, it was replaced by a
local loop iterating the array.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Jonathan Kim <jonathan.kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>