drm/i915: properly clear SSC1 bit in the pch refclock init code

Noticed by staring at intel_reg_dumper diffs. Unfortunately it does
not seem to completely fix the bug.

Still, it's good to get this right, and maybe it helps someplace else.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=47117


Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>