intel/blorp: Set needs_(dst|src)_offset for Gen4 cubemaps

intel/blorp: Set needs_(dst|src)_offset for Gen4 cubemaps

We call convert_to_single_slice so they may end up with a non-trivial
offset that needs to be taken into account.

v2 (idr): Also set needs_src_offset.  Suggested by Jason.

Fixes ES2-CTS.functional.texture.specification.basic_copyteximage2d.cube_rgba
and ES2-CTS.functional.texture.specification.basic_copytexsubimage2d.cube_rgba
on G45.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101284
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>