drm/i915: Modifying structures related to DRRS

Earlier, DRRS structures were specific to eDP (used only in intel_dp).
Since DRRS can be extended to other internal display types
(if the panel supports multiple RR), modifying structures
to be part of drm_i915_private and have a provision to add display related
structs like intel_dp.
Also, aligning with frontbuffer tracking mechanism, the new structure
contains data for busy frontbuffer bits.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>