drm/i915: IBX+ doesn't have separate vsync/hsync controls on the VGA DAC

When the PCH split occurred, hw dropped support for separate hsync and
vsync disable in the VGA DAC.  So add a PCH specific DPMS function that
just uses the port enable bit for controlling DPMS states.

Before this fix, when anything other than a full DPMS off occurred,
the VGA port would be left enabled and scanning out while all the other
heads would turn off as expected.

v2: duplicate encoder helper vtable into pch and gmch versions (Daniel)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=48491


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: s/intel_crt_dpms/gmch_crt_dpms as suggested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>