For RGB, there is no initialization code to make this work out of the box, just pin configuration.
For MCU and RGB:
The IM[1:0] bits are used to set the MCU interface to either 16 or 18 bit (01 or 11). IM2 selects either Serial (0) or parallel (1) MCU interface. So for example, IM[2:0] = 011 would mean the MCU interface is running in 18-bit serial mode.
The RCM1 bit is used to select either the MCU (0) or RGB (1) interface modes (for writing display data to frame memory).
If the RGB interface mode is selected (RCM1 = 1), then RCM2 selects either RGB interface 1 (0) or RGB interface 2 (1). So to use RGB interface 1 you would set RCM[1:0] = 10.
The differences between the two are:
RGB Interface 1 (RCM [1:0] = 10) - Writing data to frame memory is done by PCLK and the data bus when DE is High. The controller must transfer the PCLK, VS, HS and DE signals to the driver.
RGB Interface 2 (RCM [1:0] = 11) - When the DE pin is High, display data is stored directly to frame memory. Blank porch setting is done using the B5h command (see page 152 of the display driver datasheet)
In the case of using the RGB interface (RCM1 = 1) with the Serial MCU interface (IM2 = 0), you can use the SDA pin for serial communication between your microcontroller and the display driver.