i965/drm: Make alignment parameter a uint64_t.

i965/drm: Make alignment parameter a uint64_t.

Theoretically, with a 48-bit address space, we could have buffers
with an alignment of >= 4GB.  It's a bit silly, but the exec_object
structs (drm_i915_gem_exec_object2) use a __u64 for this, so we may
as well use the same type as the kernel API.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>