freedreno/a6xx: fix blend all_mrt_write_mask
Fix all_mrt_write_mask when independent_blend_enable is false. Otherwise, lrz write is always diabled with MRT when independent_blend_enable is false. This fixes a 2% perf regression for multiple gfxbench benchmarks.
Fixes: 0132c22d ("freedreno/a6xx: Don't disable LRZ for invalid channels")