drm/i915: add intel_display_suspend, v2

This is a function used to disable all crtc's. This makes it clearer
to distinguish between when mode needs to be preserved and when
it can be trashed.

Changes since v1:
- Copy power changes from intel_crtc_control.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>