graphics/gpu-firmware-amd-kmod: fix plist for BEIGE_GOBY and POLARIS12 firmwares

Pet stage-qa:

Add missed POLARIS12 firmware part (amdgpu_polaris12_32_mc_bin.ko)
Remove duplicate lines for BEIGE_GOBY firmware

Reported by: poudriere bulk -t
Approved by: portmgr blanket
Sponsored by: Serenity Cybersecurity, LLC