drm/i915/display: Convert i915_suspend into i9xx_display_sr

drm/i915/display: Convert i915_suspend into i9xx_display_sr

These save & restore functions inside i915_suspend are old display
functions to save and restore a bunch of display related registers.

Move it under display and rename accordantly. Just don't move it
entirely towards intel_display struct yet because it depends
on drm_i915_private for the IS_MOBILE.

While doing this conversion also update the MIT header using
the new SPDX ones.

v2: Fix Makefile and include (Jani)
    Removed vga and gmbus (Jani, Ville)

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241113225016.208673-2-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>