drm/i915: Keep sprite plane src rect in 16.16 format

This patch keeps intel_plane_state->src rect back
into 16.16 format.

v2:
-sprite src rect to match primary format (Matt, Daniel)

v3:
-moved a hunk from #14 to keep src rect in check & commit in tandom (Matt)

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>