vc4: Avoid emitting small immediates for UBO indirect load address guards.

vc4: Avoid emitting small immediates for UBO indirect load address guards.

The kernel will reject our shader if we emit one here, and having 4, 8, or
12 as the top end of our UBO clamp rare is enough that it's not worth
making the kernel let us.

Fixes piglit fs-const-array-of-struct and
fs-const-array-of-struct-of-array since recent GLSL linking changes made
us get this as an indirect load of a uniform, instead of a tempoary.

Cc: "13.0 17.0" <mesa-stable@lists.freedesktop.org>