Usually, you just have to add "amd_pstate=passive" (without the parenthesis) to your kernel parameters, save and close the file, and then issue sudo update-grub to rebuild your grub configuration file with the new kernel parameter that you just added. As per the Ubuntu wiki: https://wiki.ubuntu.com/Kernel/KernelBootParameters at the bottom of the page it tells you how to properly and permanently add a kernel parameter to your /etc/default/grub file.
Here is the relevant section:
Permanently Add a Kernel Boot Parameter
Log in to the system and start a terminal window (Applications->Accessories->Terminal).
In the terminal window at the $ prompt, enter the command: sudo gedit /etc/default/grub
Enter your password when prompted by [sudo].
If the file /etc/default/grub appears to be empty or does not exist, see the instructions for earlier releases above).
In the editor window, use the arrow keys to move the cursor to the line beginning with "GRUB_CMDLINE_LINUX_DEFAULT" then edit that line, adding your parameter(s) to the text inside the double-quotes after the words "quiet splash". (Be sure to add a SPACE after "splash" before adding your new parameter.) Click the Save button, then close the editor window.
In the terminal window at the $ prompt, enter the command: sudo update-grub
Restart the system.
Note: The parameter(s) you've added to the GRUB_CMDLINE_LINUX_DEFAULT line are persistent, and will be in effect for every subsequent boot session (unless you remove them by repeating the procedure above).
