st/xa: Fix regression in xa_yuv_planar_blit()

Commit "st/xa: scissor to help tilers" broke xa_yuv_planar_blit() and vmwgfx
textured video. Fix this by implementing scissors also in the yuv draw path.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Cc: Rob Clark <robclark@freedesktop.org>
Cc: "10.2 10.3" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 46537f1d03ba6de83be70ac574f633bb4342a327)