/CS is Chip Select, which is used when you connect multiple devices to the same SPI bus. Also, the "/" means that it is an active low signal, so you need to tie that pin to GND if you want your device to receive data. SDI and SCK are the data and clock lines used for SPI communication. By tying the 3 pins to +3V3 you are pretty much disabling the SPI communication port.
That display uses the R[7:0], G[7:0], B[7:0] pins to send the pixel data and the HSYNC, VSYNC and PCLK pins to control the timing of pixels and lines. All those signals need to be synced perfectly, which is why the manufacturer is using the FT813Q video controller. If you want to 'manually' control the display for experimental purposes, I'd recommend using an FPGA since you could have better control of the timing.