i965: Stop passing num_samples to intel_miptree_alloca_hiz().

The number of samples is already available in the miptree data
structure, so there's no need to pass it in.

I suspect this may fix a subtle bug because in one case
(intel_renderbuffer_update_wrapper) we were always passing zero for
num_samples, even though the buffer in question was not guaranteed to
be single-sampled.  But I wasn't able to find a failing test case.