radeonsi: enable dithered alpha-to-coverage for better quality

same as AMDVLK.

GL_NV_alpha_to_coverage_dither_control allows controlling this behavior.
The default is implementation-dependent.

src/gallium/drivers/radeonsi/si_state.c

@@ -474,10 +474,11 @@ static void *si_create_blend_state_mode(struct pipe_context *ctx,
 
 	si_pm4_set_reg(pm4, R_028B70_DB_ALPHA_TO_MASK,
 		       S_028B70_ALPHA_TO_MASK_ENABLE(state->alpha_to_coverage) |
-		       S_028B70_ALPHA_TO_MASK_OFFSET0(2) |
-		       S_028B70_ALPHA_TO_MASK_OFFSET1(2) |
-		       S_028B70_ALPHA_TO_MASK_OFFSET2(2) |
-		       S_028B70_ALPHA_TO_MASK_OFFSET3(2));
+		       S_028B70_ALPHA_TO_MASK_OFFSET0(3) |
+		       S_028B70_ALPHA_TO_MASK_OFFSET1(1) |
+		       S_028B70_ALPHA_TO_MASK_OFFSET2(0) |
+		       S_028B70_ALPHA_TO_MASK_OFFSET3(2) |
+		       S_028B70_OFFSET_ROUND(1));
 
 	if (state->alpha_to_coverage)
 		blend->need_src_alpha_4bit |= 0xf;