drm/amd/display: Ensure array index tg_inst won't be -1

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Ensure array index tg_inst won't be -1 [WHY & HOW] tg_inst will be a negative if timing_generator_count equals 0, which should be checked before used. This fixes 2 OVERRUN issues reported by Coverity.