llvmpipe: initialize llvmpipe->dirty with LP_NEW_SCISSOR

llvmpipe: initialize llvmpipe->dirty with LP_NEW_SCISSOR

If llvmpipe_set_scissor_states() is never called, we still need to be sure
that derived scissor/clip state is updated.  As of commit 743ad599a97d09b1
that function might not be called.

Fixes regressed Piglit gl-1.0-scissor-offscreen -fbo -auto test.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101709
Fixes: 743ad599a97 ("st/mesa: don't set 16 scissors and 16 viewports
if they're unused")
Cc: "17.2" <mesa-stable@lists.freedesktop.org>