waveshare2in13v2: Document command 0x37 and always apply it

The datasheets of the comparable SSD1680, SSD1675B and GDEH0213B73
controllers describe this command as "Write register for display
option".

Signed-off-by: Michael Hanselmann <public@hansmi.ch>

waveshare2in13v2/controller.go

@@ -63,11 +63,10 @@ func configDisplayMode(ctrl controller, mode PartialUpdate, lut LUT) {
 	ctrl.sendCommand(writeLutRegister)
 	ctrl.sendData(lut[:70])
 
-	if mode == Partial {
+	ctrl.sendCommand(writeDisplayOptionRegister)
-		// Undocumented command used in vendor example code.
-		ctrl.sendCommand(0x37)
 	ctrl.sendData([]byte{0x00, 0x00, 0x00, 0x00, 0x40, 0x00, 0x00})
 
+	if mode == Partial {
 		ctrl.sendCommand(displayUpdateControl2)
 		ctrl.sendData([]byte{
 			displayUpdateEnableClock |

waveshare2in13v2/controller_test.go

@@ -91,6 +91,7 @@ func TestConfigDisplayMode(t *testing.T) {
 				{cmd: writeVcomRegister, data: []byte{0x55}},
 				{cmd: borderWaveformControl, data: []byte{0x03}},
 				{cmd: writeLutRegister, data: bytes.Repeat([]byte{'F'}, 70)},
+				{cmd: 0x37, data: []byte{0x00, 0x00, 0x00, 0x00, 0x40, 0x00, 0x00}},
 			},
 		},
 		{

waveshare2in13v2/waveshare213v2.go

@@ -37,6 +37,7 @@ const (
 	setDummyLinePeriod             byte = 0x3A
 	setGateTime                    byte = 0x3B
 	borderWaveformControl          byte = 0x3C
+	writeDisplayOptionRegister     byte = 0x37
 	setRAMXAddressStartEndPosition byte = 0x44
 	setRAMYAddressStartEndPosition byte = 0x45
 	setRAMXAddressCounter          byte = 0x4E