drm/i915: Calculate the sprite WM based on the source width instead of the destination width

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>