Revert "drm/i915/ilk: Don't disable SSC source if it's in use"

Revert "drm/i915/ilk: Don't disable SSC source if it's in use"

This reverts commit f165d2834ceb3d5c29bebadadc27629bebf402ac.

It breaks one of our CI systems. Quoting from Ville:

[   13.100979] [drm:ironlake_init_pch_refclk] has_panel 1 has_lvds 1 has_ck505 0 using_ssc_source 1
[   13.101413] ------------[ cut here ]------------
[   13.101429] kernel BUG at drivers/gpu/drm/i915/intel_display.c:8528!

"which is the 'BUG_ON(val != final)' at the end of ironlake_init_pch_refclk()."

Cc: stable@vger.kernel.org
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lyude <cpaul@redhat.com>
Cc: marius.c.vlad@intel.com
References: https://www.spinics.net/lists/dri-devel/msg109557.html
Acked-by: Lyude <cpaul@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>