vc4: Fix viewport handling in the uniforms upload.

vc4: Fix viewport handling in the uniforms upload.

I had the right viewports in vc4_emit.c, but grabbed the wrong values in
the uniform setup, so primitives would claim to be in the wrong parts of
the screen.  (The vc4_emit.c state looks like it just decides how big the
clipping guardband is).

This gets fbo-viewport closer to working (which still has the problem that
the HW is always guard-band clipping), and fixes inverted FBO rendering in
general.