sna/video: Try to use hw scaling with SKL+ sprites

SKL reintroduced plane scaling once more. Let's try to make use of it.

The one annoying caveat is that you can't do colorkeying and scaling at
the same time :(  For now we'll leave the choice of colorkey vs.
scaling to the user via that XV_ALWAYS_ON_TOP attribute.

One possible idea for improving the situation would be to add support
for autopaint colorkey, and automatically disable the colorkey whenever
the window is not obscured by anything and autopaint colorkey is enabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>