vmx: fix pix_multiply for ppc64le

vmx: fix pix_multiply for ppc64le

vec_mergeh/l operates differently for BE and LE, because of the order of
the vector elements (l->r in BE and r->l in LE).
To fix that, we simply need to swap between the input parameters, in case
we are working in LE.

v2:

- replace _LITTLE_ENDIAN with WORDS_BIGENDIAN for consistency
- fixed whitespaces and indentation issues

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>