This commit was borrowed from uxa driver contributed by Eric.
    commit number is e0066e77e026b0dd0daa0c3765473c7d63aa6753. commit log paste as
    below:
    We were clipping each span against the bounds of the clip, throwing
    out the span early if it was all clipped, and then walked the clip box
    clipping against each of the cliprects.  We would expect spans to
    typically be clipped against one box, and not thrown out, so we were
    not saving any work there.  For multiple cliprects, we were adding
    work.  Only for many spans clipped entirely out of a complicated clip
    region would it have saved work, and it clearly didn't save bugs as
    evidenced by the many fix attempts here.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>