drm: Promote drm_mm alignment to u64

In places (e.g. i915.ko), the alignment is exported to userspace as u64
and there now exists hardware for which we can indeed utilize a u64
alignment. As such, we need to keep 64bit integers throughout when
handling alignment.

Testcase: igt/drm_mm/align64
Testcase: igt/gem_exec_alignment
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-22-chris@chris-wilson.co.uk