drm/i915: Readout initial hw mode, v2.

Atomic requires a mode blob when crtc_state->enable is true, or
you get a huge warn_on.

With a few tweaks the mode we read out from hardware could be used
as the real mode without a modeset, but this requires too much
testing, so for now force a modeset the first time the mode blob's
updated.

This preserves the old behavior, because previously we never set
the initial mode, which always meant that a modeset happened
when the mode was first set.

Changes since v1:
- Add a description in intel_modeset_readout_hw_state of how the
  recalculation is done.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>