drm/i915: Disable/enable planes as the first/last thing during modeset on HSW

Refactor the plane enabling/disabling into helper functions and move
the calls to happen as the first thing during .crtc_disable, and the
last thing during .crtc_enable.

Those are the two clear points where we are sure that the pipe is
actually running regardless of the encoder type or hardware
generation.

v2: Made by Paulo:
  Remove the code touching everything but the Haswell functions. We
  need this change on Haswell right now since it fixes a FIFO underrun
  that we get on pipe A while we enable pipe B (see the workaround
  notes on the Haswell mode set sequence documentation). We can bring
  back the code to gens 2-7 later, once they're tested.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>