drm/i915: abstract intel_panel_sanitize_ssc() from intel_modeset_init()

The code is too specific and detailed to have open in a high level
function. Abstract away. As a drive-by improvement switch to using
enableddisabled() in logging and git rid of a redundant !!. No
functional changes.

v2: drop the !! while at it too (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920185421.17822-4-jani.nikula@intel.com