drm/i915: add opregion function to notify bios of encoder enable/disable

The bios interface seems messy, and it's hard to tell what the bios
really wants. At first, only add support for DDI based machines (hsw+),
and see how it turns out.

The spec says to notify prior to power down and after power up. It is
unclear whether it makes a difference.

v2:
 - squash notification function and callers patches together (Daniel)
 - move callers to haswell_crtc_{enable,disable} (Daniel)
 - rename notification function (Chris)

v3:
 - separate notification function and callers again, as it's not clear
   whether the display power state notification is the right thing to do
   after all

v4: per Paulo's review:
 - drop LVDS
 - WARN on unsupported encoder types

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>