drm/i915: fix pfit regression for non-autoscaled resolutions

I.e. for letter/pillarboxing. For those cases we need to adjust the
mode a bit, but Jesse gmch pfit refactoring in

commit 2dd24552


Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Apr 25 12:55:01 2013 -0700

    drm/i915: factor out GMCH panel fitting code and use for eDP v3

broke that by reordering the computation of the gmch pfit state with
the block of code that prepared the adjusted mode for it and told the
modeset core not to overwrite the adjusted mode with default settings.

We might want to switch around the core code to just fill in defaults,
but this code predates the pipe_config modeset rework. And in the old
crtc helpers we did not have a suitable spot to do this.

Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Hans de Bruin <jmdebruin@xmsnet.nl>
Reported-and-tested-by: Hans de Bruin <jmdebruin@xmsnet.nl>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>