PM: GPU: Disable GPU OC

We already have a working GPU OC interface.
This option OMAP4430_GPU_OVERCLOCK causes issues in lopgs and might be responsible for kexec rejects on TARGA and EDISON:

[    1.775329,1] omap4_gpu_opp_enable: no gpu_dev, did not enable f=153600000
[    1.775390,1] omap4_gpu_opp_enable: no gpu_dev, did not enable f=384000000
[    1.775390,1] omap4_gpu_opp_enable: no gpu_dev, did not enable f=416000000