headers: the uint*_t vs. __u* discrepancy in amdgpu_drm is fixed

This was already done in commit 3dc002df ("amdgpu: sync amdgpu_drm.h
with kernel 4.11-rc2"), now update the README accordingly.

Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>