i965/gen5+: Fix incorrect miptree layout for non-power-of-two cubemaps.

i965/gen5+: Fix incorrect miptree layout for non-power-of-two cubemaps.

For power-of-two sizes, h0 == mt->height0 since it's already a multiple
of two.  However, for NPOT, they're different; h1 should be computed
based on the original size.

Fixes piglit test "cubemap npot" and oglconform test "textureNPOT".

NOTE: This is a candidate for stable release branches.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>