Display: display_clear() is a common command now.

Before control commands were queued up, this was different for
each display.

display.c

@@ -29,6 +29,14 @@ volatile uint8_t displaybuf[BUFSIZE];
 
 #include "delay.h"
 
+/**
+  Queue up a clear screen command. Very cheap operation on some displays, like
+  the HD44780, rather expensive on others, like the SSD1306.
+*/
+void display_clear(void) {
+  display_writechar((uint8_t)low_code_clear);
+}
+
 /**
   Prints a character at the current cursor position.
 

display_hd44780.c

@@ -37,13 +37,6 @@ void display_init(void) {
   displaybus_write(0x0C, parallel_4bit_instruction);
 }
 
-/**
-  Queue up a clear screen command. Cheap operation on this display.
-*/
-void display_clear(void) {
-  display_writechar((uint8_t)low_code_clear);
-}
-
 /**
   Sets the cursor to the given position.
 

display_ssd1306.c

@@ -114,14 +114,6 @@ void display_init(void) {
   }
 }
 
-/**
-  Queue up a clear screen command. Be careful, this is an expensive operation
-  on this display.
-*/
-void display_clear(void) {
-  display_writechar((uint8_t)low_code_clear);
-}
-
 /**
   Sets the cursor to the given position.