ST7789: Add RAMCTRL mentioned in #1040.

drivers/st7789/st7789.cpp

@@ -23,6 +23,7 @@ namespace pimoroni {
     TEON      = 0x35,
     MADCTL    = 0x36,
     COLMOD    = 0x3A,
+    RAMCTRL   = 0xB0,
     GCTRL     = 0xB7,
     VCOMS     = 0xBB,
     LCMCTRL   = 0xC0,
@@ -79,6 +80,12 @@ namespace pimoroni {
     command(reg::PWCTRL1, 2, "\xa4\xa1");
     command(reg::FRCTRL2, 1, "\x0f");
 
+    // As noted in https://github.com/pimoroni/pimoroni-pico/issues/1040
+    // this is required to avoid a weird light grey banding issue with low brightness green.
+    // The banding is not visible without tweaking gamma settings (GMCTRP1 & GMCTRN1) but
+    // it makes sense to fix it anyway.
+    command(reg::RAMCTRL, 2, "\x00\xc0");
+
     if(width == 240 && height == 240) {
       command(reg::GCTRL, 1, "\x14");
       command(reg::VCOMS, 1, "\x37");