drm/i915: clean up crtc timings computation

In the old days of the crtc helpers we've only had the encoder and
crtc ->mode_fixup callbacks. So when the lvds connector wanted to
adjust the crtc timings it had to set a driver-private mode flag to
tell the crtc mode fixup code to not overwrite them with the generic
ones.

When converting things to the new infrastructure I've kept the entire
logic and only moved the flag to pipe_config->timings_set. But this
logic is pretty tricky and already caused regressions:

commit 21d8a475
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jul 12 08:07:30 2013 +0200

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

So take advantage of the flexibility our own modeset infrastructure
affords us and prefill default crtc timings. This allows us to rip out
->timings_set. Note that we overwrite things again when retrying the
pipe config computation due to bandwidth constraints to avoid bogus
crtc timings if the encoder only does relative adjustments (which is
how the pfit code works). Only a theoretical concern though since
platforms where we retry (pch-split platforms) do not need
adjustements (since only the old gmch pfit needs that). But let's
better be safe than sorry.

Since we now initialize the crtc timings before calling the
encoder->compute_config functions the crtc initialization in the gmch
pfit code is now redudant and so can be removed.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Add a paragraph to the commit message to explain why we can
ditch the crtc timings initialization call from the gmch pfit code, to
answer a question from Rodrigo's review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>