Disable gfx hw accel on i.MX6 with mildenhall-compositor

After increasing the available CMA memory on the i.MX6 SABRElite boards and thus making gfx hw acceleration fully functional the Mildenhall compositor has been badly flickering, while the AGL compositor works fine.

Since the Mildenhall compositor is on its way out, revert to the previous situation by forcefully disabling acceleration in Mesa when using it.