Fix use of ISSI driver config before init (#16493)

When helix/rev2 was using split_common, it didn't work properly and I couldn't type on the right hand side.

The cause is that the following code, added in 0.16.0 to `quantum/keyboard.c`, does not include `quantum/split_common/split_util.h` but instead includes `keyboards/helix/rev2/split_util.h`. Therefore, `split_pre_init()/split_post_init()` in `quantum/split_common/split_util.c` was not called.

```c
  #ifdef SPLIT_KEYBOARD
  #    include "split_util.h"
  #endif
```

keyboards/helix/rev2/custom/split_util.h

@@ -1,5 +1,10 @@
 #pragma once
 
+#ifdef SPLIT_KEYBOARD
+#   error This is helix local split_util.h.
+#   error This header file is used only when SPLIT_KEYBOARD=no.
+#endif
+
 #include <stdbool.h>
 #include "eeconfig.h"
 
@@ -14,7 +19,3 @@ void split_keyboard_setup(void);
 bool is_helix_master(void);
 
 void matrix_master_OLED_init (void);
-
-// stubs as this is handled by legacy code
-static inline void split_pre_init(void){}
-static inline void split_post_init(void){}

keyboards/helix/rev2/local_features.mk

@@ -52,9 +52,10 @@ ifneq ($(strip $(SPLIT_KEYBOARD)), yes)
 
   CUSTOM_MATRIX = yes
 
-  SRC += rev2/matrix.c
-  SRC += rev2/split_util.c
-  SRC += rev2/split_scomm.c
+  SRC += rev2/custom/matrix.c
+  SRC += rev2/custom/split_util.c
+  SRC += rev2/custom/split_scomm.c
+  KEYBOARD_PATHS += $(HELIX_TOP_DIR)/rev2/custom
 endif
 
 ########

tmk_core/protocol/arm_atsam/i2c_master.c

@@ -297,6 +297,9 @@ uint8_t I2C3733_Init_Drivers(void) {
     if (gcr_actual > LED_GCR_MAX) gcr_actual = LED_GCR_MAX;
     gcr_desired = gcr_actual;
 
+    void issi3733_prepare_arrays(void);
+    issi3733_prepare_arrays();
+
     // Set up master device
     i2c_led_send_CRWL(0);
     i2c_led_select_page(0, 3);

tmk_core/protocol/arm_atsam/main_arm_atsam.c

@@ -300,8 +300,6 @@ int main(void) {
     i2c1_init();
 #endif // RGB_MATRIX_ENABLE
 
-    matrix_init();
-
     USB_Hub_init();
 
     DBGC(DC_MAIN_UDC_START_BEGIN);

tmk_core/protocol/arm_atsam/md_rgb_matrix.c

@@ -206,6 +206,12 @@ void gcr_compute(void) {
 }
 
 void issi3733_prepare_arrays(void) {
+    static bool s_init = false;
+    if (s_init) {
+        return;
+    }
+    s_init = true;
+
     memset(issidrv, 0, sizeof(issi3733_driver_t) * ISSI3733_DRIVER_COUNT);
 
     int     i;