drm/i915: Move w/a 0477/WaDisableIPC:skl into intel_init_ipc()

Move the w/a to disable IPC on SKL closer to the actual code
that implements IPS. Otherwise I just end up confused as to
what is excluding SKL from considerations.

IMO this makes more sense anyway since the hw does have the
feature, we're just not supposed to use it.

And this also makes us actually disable IPC in case eg. the
BIOS enabled it when it shouldn't have.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190503173807.10834-3-ville.syrjala@linux.intel.com