ppc: pixblockdsp: do unaligned block accesses correctly again

This was broken by the following Libav commit:
4c387c7 ppc: dsputil: do unaligned block accesses correctly

The following tests fail due to this:
fate-checkasm
fate-vsynth1-dnxhd-2k-hr-hq fate-vsynth1-dnxhd-edge1-hr
fate-vsynth1-dnxhd-edge2-hr fate-vsynth1-dnxhd-edge3-hr
fate-vsynth1-dnxhd-hr-sq-mov fate-vsynth1-dnxhd-hr-hq-mov
fate-vsynth2-dnxhd-2k-hr-hq fate-vsynth2-dnxhd-edge1-hr
fate-vsynth2-dnxhd-edge2-hr fate-vsynth2-dnxhd-edge3-hr
fate-vsynth2-dnxhd-hr-sq-mov fate-vsynth2-dnxhd-hr-hq-mov
fate-vsynth3-dnxhd-2k-hr-hq fate-vsynth3-dnxhd-edge1-hr
fate-vsynth3-dnxhd-edge2-hr fate-vsynth3-dnxhd-edge3-hr
fate-vsynth3-dnxhd-hr-sq-mov fate-vsynth3-dnxhd-hr-hq-mov

Fixes trac ticket #5508.

Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
