drm/i915: Enable PCH FIFO underruns later on ILK/SNB/IVB

We get spurious PCH FIFO underruns if we enable the reporting too soon
after enabling the crtc. Move it to be the last step, after the encoder
enable. Additionally we need an extra vblank wait, otherwise we still
get the underruns. Presumably the pipe/fdi isn't yet fully up and running
otherwise.

For symmetry, disable the PCH underrun reporting as the first thing,
just before encoder disable, when shutting down the crtc.

v2: Do the PCH underrun enable unconditionally (Jani, Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1446225627-10809-1-git-send-email-ville.syrjala@linux.intel.com