drm/i915: Don't update staged config during force restore modesets

The force restore path relies on the staged config to preserve the
configuration used before a suspend/resume cycle. The update done to it
in intel_modeset_fixup_state() would cause that information to be lost
after the first modeset, making it impossible to restore the modes for
pipes B and C.

References: https://bugs.freedesktop.org/show_bug.cgi?id=90468


Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>