drm/i915: Power gating display wells during i915_pm_suspend

On VLV, after i915_pm_suspend display power wells are staying
power ungated. So, after initiating mem sleep "echo mem > /sys/power/state"
Display is staing D0 State. There might be better way/place to power gate
these wells. Also, we need to make sure that if wells are power gated due to
DPMS OFF sequence, they need not be turned off by i915_pm_suspend again.

v2: Extracted helper for intel_crtc_disable and power gating CRTC power wells.
[Daniel]

Cc: Imre Deak <imre.deak@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Change-Id: I34c80da66aa24c423a5576c68aa1f3a8d0f43848
Signed-off-by: Borun Fu <borun.fu@intel.com>
Signed-off-by: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>