drm/amdkfd: Remove skiping userptr buffer mapping when mmu notifier marks it as invalid

mmu notifier does not always hold mm->sem during call back. That causes
a race condition between kfd userprt buffer mapping and mmu notifier
which leds to gpu shadder or SDMA access userptr buffer before it has been
mapped to gpu VM. Always map userptr buffer to avoid that though it may make
some userprt buffers mapped two times.

Suggested-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Xiaogang Chen <xiaogang.chen@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>