Add a __drm_to_display() conversion function to hide the to_i915() usage
and the implicit dependency on i915_drv.h from intel_display_types.h.
The goal is for this implementation to be a transitional helper
only. One idea I've floated around in the past would be to require a
struct intel_display pointer member to be placed right after struct
drm_device member in struct drm_i915_private and struct xe_device
[1][2].