drm/i915/overlay: Abstract buffer (un)pinning

drm/i915/overlay: Abstract buffer (un)pinning

Make the buffer (un)pinning a bit more abstract so that
the display side of the code doesn't need to know about i915
specific things (i915_ggtt_offset() and i915_vma_unpin()).

In preparation for the full parent vs. display driver split
we'll also pass in the drm_device to these functions, although
not strictly needed yet.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226100738.29997-12-ville.syrjala@linux.intel.com