This is defined to always clear the entire surface(s) specified,
regardless of scissor state.. mesa/st will turn scissored clears
into a draw.  So rip about a bunch of unnecessary machinery.

Also remove a comment that was obsolete since using u_blitter to
turn clear into draw (for the cases where there isn't a hw blitter
fast-path).

Signed-off-by: Rob Clark <robdclark@gmail.com>