drm/amdgpu: Merge amdkfd into amdgpu

Since KFD is only supported by single GPU driver, it makes sense to merge
amdgpu and amdkfd into one module. This patch is the initial step: merge
Kconfig and Makefile.

v2: also remove kfd from drm Kconfig

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