drm/i915: use compute_config in set_config v4

This will allow us to consult more info before deciding whether to flip
or do a full mode set.

v2:
  - don't use uninitialized or incorrect pipe masks in set_config
    failure path (Ander)
v3:
  - fixup for pipe_config changes in compute_config (Jesse)
v4:
  - drop spurious hunk in force restore path (Ander)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>