AMD Radeon HD 6770M PCIe support on OS X EI Captain for Mac pro late 2011

I have the Graphics card AMD Radeon HD 6770 PCIe, but when I checked in the display, it is showing that it is using built-in-display, only. How can I use and enable AMD Radeon card. I am not getting any support or device driver or any program which can check it. Now a days, I am getting issues that my mac book pro late 2011 gets heated and then it's graphics turning int lines on the display,probably it is due to using the built in display.

Please help me to use Radeon display and to check it's performance.