drm/i915: extract intel_set_config_compute_mode_changes

This computes what exactly changed in the modeset configuration, i.e.
whether a full modeset is required or only an update of the
framebuffer base address or no change at all.

In the future we might add more checks for e.g. when only the output
mode changed, so that we could do a minimal modeset for outputs that
support this. Like the lvds/eDP panels where we only need to update
the panel fitter.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>