i965/miptree: Use num_samples of 1 instead of 0 for single-sampled

i965/miptree: Use num_samples of 1 instead of 0 for single-sampled

Patch moves "assert(brw->num_samples <= 16)" from
emit_3dstate_multisample2() to upload_multisample_state(). Latter
is the only caller of the former and passes "brw->num_samples"
as argument. Therefore it is clearer to assert in the caller.

Possible bug fix in genX(emit_3dstate_multisample2) which
doesn't have a case for num_samples == 0 in the switch
statement.

It should be noted that intel_miptree_map()/unmap() now checks
additionally for "mt->surf.samples == 1" in order to support gen6
stencil which is already transitioned to ISL. This will go away in
next patch when native miptrees start to use isl_surf::samples as
well.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>