i.MX31: Restore screen clearing in DVFS/DPTC debug screen

Somewhere along the line the screen stopped being cleared prior to
writing new text on each frame, which left visible bits of
previously-displayed text when it changed.

Change-Id: I344e03c234daa77f4e64ed89281c40db887e4498

firmware/target/arm/imx31/debug-imx31.c

@@ -260,7 +260,6 @@ bool __dbg_dvfs_dptc(void)
     unsigned long ltw[4];
     bool ltwassert[4];
 
-    lcd_clear_display();
     lcd_setfont(FONT_SYSFIXED);
 
     dvfs_mask = dvfs_level_mask();
@@ -275,6 +274,8 @@ bool __dbg_dvfs_dptc(void)
 
         int button = button_get_w_tmo(HZ/10);
 
+        lcd_clear_display();
+
         if (dvfs_wp < 0)
             strcpy(buf, "Auto");
         else