Hi Pankti,
The way to interface an 18-bit LCD is to set the LCD controller in 24-bit (RGB-888) mode and connect the LCD display to the highest 6 bits of each color. The free LCD_DATA pins can be pinmuxed to another mode, but take care to check the AM335X Errata Usage Note 3.1.1 - these will be LCD_DATA[23:18].