drm/i915/display: add struct drm_device to struct intel_display conversion function

drm/i915/display: add struct drm_device to struct intel_display conversion function

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].

[1] https://lore.kernel.org/r/7777ff70e2be0663de4398aa6f75f0c54146cbfc.1709727127.git.jani.nikula@intel.com
[2] https://lore.kernel.org/r/0b9459da6c8cba0f74bf2781d69182fa6801cd97.1709727127.git.jani.nikula@intel.com

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cbbf26fa58ef662946303c972b1a1ff1547ddcfe.1732104170.git.jani.nikula@intel.com