Hi Akshat,
Please refer to figures 41 and 42 of the GD3162 datasheet. After setting BIST=1, INTA & INTB pin will go low and BIST will be run. It can take up to 15ms for BIST to complete. When BIST is complete, INTA and INTB will go back high. The code needs to wait for this indication to read SPI. SPI read during BIST is not allowed. After BIST has completed, the code should write BIST bit to 0 and check either the state of INTA/INTB or the BIST_ERR flag to know if it was successful.
BRs, Tomas