Using the destination width in the sprite WM calculations isn't correct.
We should be using the source width.

Note: This doesn't affect hsw since it does not support sprite
scaling.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Add review note from Paulo to the commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>