drm/amdkfd: Use real device for messages

kfd_chardev() doesn't provide much useful information in dev_... messages
on multi-GPU systems because there is only one KFD device, which doesn't
correspond to any particular GPU. Use the actual GPU device to indicate
the GPU that caused a message.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>