drm/i915: Don't check gmch state on inherited configs

... our current modeset code isn't good enough yet to handle this. The
scenario is:

1. BIOS sets up a cloned config with lvds+external screen on the same
pipe, e.g. pipe B.

2. We read out that state for pipe B and assign the gmch_pfit state to
it.

3. The initial modeset switches the lvds to pipe A but due to lack of
atomic modeset we don't recompute the config of pipe B.

-> both pipes now claim (in the sw pipe config structure) to use the
gmch_pfit, which just won't work.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74081


Tested-by: max <manikulin@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>