Fix broken cubemap mipmap layout (the cubemap.c demo was segfaulting).

Fix broken cubemap mipmap layout (the cubemap.c demo was segfaulting).

In i915_miptree_layout() change the width, height parameters that are passed
to intel_miptree_set_level_info().  As it was, the width, height values were
larger than the source image dimensions and we segfaulted in memcpy() when
copying the original texture data into the texture buffer region.
This fix should probably be checked by someone more familiar with the code (Keith?)