Reduce the module parameter to enable or disable.

The link stand by vs full link off was used only once.

And it was actually masking another bug fixed by commit
'84bb2916 ("drm/i915/psr: Check for SET_POWER_CAPABLE
bit at PSR init time.")'

So, let's remove these options for now. End goal is to
fully remove the mod param, moving it to a debugfs
interface in upcoming patches.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Tarun Vyas <tarun.vyas@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712052715.8177-1-rodrigo.vivi@intel.com