Exactly, the D/C is not needed with I2C.
You could use u8glib (Google Code Archive - Long-term storage for Google Code Project Hosting.).
There is a SPI constructors for your display:
U8GLIB_SSD1306_128X64(sck, mosi, cs, a0, reset)
(with a0 = D/C)
As arguments put in the pin numbers of those ports to which the corresponding display signal is connected.
Oliver