Let laser/spindle use free AVR servo timers (#19011)

Marlin/src/HAL/AVR/ServoTimers.h

@@ -60,9 +60,11 @@
 #if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
   //#define _useTimer1
   #define _useTimer3
-  #define _useTimer4
+  #if NUM_SERVOS > SERVOS_PER_TIMER
-  #if !HAS_MOTOR_CURRENT_PWM
+    #define _useTimer4
-    #define _useTimer5 // Timer 5 is used for motor current PWM and can't be used for servos.
+    #if !HAS_MOTOR_CURRENT_PWM && SERVOS > 2 * SERVOS_PER_TIMER
+      #define _useTimer5 // Timer 5 is used for motor current PWM and can't be used for servos.
+    #endif
   #endif
 #elif defined(__AVR_ATmega32U4__)
   #define _useTimer3

Marlin/src/HAL/AVR/inc/SanityCheck.h

@@ -36,9 +36,10 @@
  * Sanity checks for Spindle / Laser PWM
  */
 #if ENABLED(SPINDLE_LASER_PWM)
+  #include "../ServoTimers.h"   // Needed to check timer availability (_useTimer3)
   #if SPINDLE_LASER_PWM_PIN == 4 || WITHIN(SPINDLE_LASER_PWM_PIN, 11, 13)
     #error "Counter/Timer for SPINDLE_LASER_PWM_PIN is used by a system interrupt."
-  #elif NUM_SERVOS > 0 && (WITHIN(SPINDLE_LASER_PWM_PIN, 2, 3) || SPINDLE_LASER_PWM_PIN == 5)
+  #elif NUM_SERVOS > 0 && defined(_useTimer3) && (WITHIN(SPINDLE_LASER_PWM_PIN, 2, 3) || SPINDLE_LASER_PWM_PIN == 5)
     #error "Counter/Timer for SPINDLE_LASER_PWM_PIN is used by the servo system."
   #endif
 #elif defined(SPINDLE_LASER_FREQUENCY)