drm/i915/vblank: drop unnecessary i915 local variable

Use struct intel_display where possible.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c925fd0a56ba0a9183b62b7cc0ead0e37264f024.1730144869.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>