radeonsi: don't ignore pitch for imported textures

Cc: 17.2 17.3 <mesa-stable@lists.freedesktop.org>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
(cherry picked from commit 022c5b22)
[Emil Velikov: attribute for lack of slice_size_dw]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

Conflicts:
	src/gallium/drivers/radeon/r600_texture.c