i965/clip: Removing scissor atom

i965/clip: Removing scissor atom

Now that we no longer use ctx->DrawBuffer->_Xmin and related fields to
program the screen-space viewport extents, we don't depend on any
scissoring state.  So we can drop the +_NEW_SCISSOR dependency.

On GEN8, a change in scissor state does not effect anything for the
clipper/sf hardware state. The hardware will always do the right thing
once the viewport extents are programmed. We can therefore remove the
unecessary state emission.

Ken originally spotted this.

v2: Reword the commit message. Remove spurious hunk.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>