drm/amdkfd: fix build, select MMU_NOTIFIER

drm/amdkfd: fix build, select MMU_NOTIFIER

When CONFIG_MMU_NOTIFIER is not enabled, struct mmu_notifier has an
incomplete type definition, which causes build errors.

../drivers/gpu/drm/amd/amdkfd/kfd_priv.h:607:22: error: field 'mmu_notifier' has incomplete type
../include/linux/kernel.h:979:32: error: dereferencing pointer to incomplete type
../include/linux/kernel.h:980:18: error: dereferencing pointer to incomplete type
../drivers/gpu/drm/amd/amdkfd/kfd_process.c:434:2: error: implicit declaration of function 'mmu_notifier_unregister_no_release' [-Werror=implicit-function-declaration]
../drivers/gpu/drm/amd/amdkfd/kfd_process.c:435:2: error: implicit declaration of function 'mmu_notifier_call_srcu' [-Werror=implicit-function-declaration]
../drivers/gpu/drm/amd/amdkfd/kfd_process.c:438:21: error: variable 'kfd_process_mmu_notifier_ops' has initializer but incomplete type
../drivers/gpu/drm/amd/amdkfd/kfd_process.c:439:2: error: unknown field 'release' specified in initializer
../drivers/gpu/drm/amd/amdkfd/kfd_process.c:439:2: warning: excess elements in struct initializer [enabled by default]
../drivers/gpu/drm/amd/amdkfd/kfd_process.c:439:2: warning: (near initialization for 'kfd_process_mmu_notifier_ops') [enabled by default]
../drivers/gpu/drm/amd/amdkfd/kfd_process.c:534:2: error: implicit declaration of function 'mmu_notifier_register' [-Werror=implicit-function-declaration]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>