vc4,v3d: Remove use of nir_intrinsic_load_alpha_ref_float
On !2213 (merged) "lowering passes from Zink" a the lowering pass nir_lower_alpha_test
can avoid using nir_intrinsic_load_alpha_ref_float. As v3d and vc4 can take advantage of the new approach this was enabled, but it generated regressions reported at #1973 (closed) "[regression][bisected] lowering passes from Zink break v3d alphatest test on piglit".
This has been temporally fixed with a revert on !2399 (merged) of the v3d/vc4 enablement patches, but we should fix so the new pass could be used.