v3d: Don't try to use the TFU blit path if a scissor is enabled.

We'll need to do a render-based blit for scissors, since the TFU (as seen
in this conditional) can only update a whole surface.

Fixes: 976ea90b ("v3d: Add support for using the TFU to do some blits.")
Fixes piglit fbo-scissor-blit.

(cherry picked from commit 4c70f276)