ir3: Use correct immediate size for constlen calculation

"size" is the allocated size of the array, not the number of immediates
actually used. We could wind up returning a too-large constlen, larger
than 512, and since the binning variant uses the non-binning variant's
constlen as it's max_const we could make binning variants use c512.x and
crash when encoding.

Fixes: 86f3c0c4c2 ("ir3: simplify constlen calculation")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40961>

src/freedreno/ir3/ir3_shader.h

@@ -1085,7 +1085,7 @@ static inline unsigned
 ir3_constlen(const struct ir3_shader_variant *v)
 {
    return ir3_const_state(v)->allocs.max_const_offset_vec4 +
-          v->imm_state.size / 4;
+          DIV_ROUND_UP(v->imm_state.count, 4);
 }
 
 static inline unsigned