freedreno/a6xx: Workaround early preamble HW bug

Port of the previous commit.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27462>

src/gallium/drivers/freedreno/a6xx/fd6_emit.cc

@@ -959,6 +959,25 @@ fd6_emit_restore(struct fd_batch *batch, struct fd_ringbuffer *ring)
    OUT_PKT4(ring, REG_A6XX_SP_PS_TP_BORDER_COLOR_BASE_ADDR, 2);
    OUT_RELOC(ring, bcolor_mem, 0, 0, 0);
 
+   /* There is an optimization to skip executing draw states for draws with no
+    * instances. Instead of simply skipping the draw, internally the firmware
+    * sets a bit in PC_DRAW_INITIATOR that seemingly skips the draw. However
+    * there is a hardware bug where this bit does not always cause the FS
+    * early preamble to be skipped. Because the draw states were skipped,
+    * SP_FS_CTRL_REG0, SP_FS_OBJ_START and so on are never updated and a
+    * random FS preamble from the last draw is executed. If the last visible
+    * draw is from the same submit, it shouldn't be a problem because we just
+    * re-execute the same preamble and preambles don't have side effects, but
+    * if it's from another process then we could execute a garbage preamble
+    * leading to hangs and faults. To make sure this doesn't happen, we reset
+    * SP_FS_CTRL_REG0 here, making sure that the EARLYPREAMBLE bit isn't set
+    * so any leftover early preamble doesn't get executed. Other stages don't
+    * seem to be affected.
+    */
+   if (screen->info->a6xx.has_early_preamble) {
+      WRITE(REG_A6XX_SP_FS_CTRL_REG0, 0);
+   }
+
    if (!batch->nondraw) {
       trace_end_state_restore(&batch->trace, ring);
    }