r300: Fix cliprect emit

This makes our handling of cliprects sane. drm_clip_rect always has exclusiv
bottom-right corners, but the hardware expects inclusive bottom-right corner
so we adjust this here.

This complements Michel Daenzer's commit 57aea290e1e0a26d1e74df6cff777eb9f03
to Mesa. See also http://bugs.freedesktop.org/show_bug.cgi?id=16123

Signed-off-by: Dave Airlie <airlied@redhat.com>