drm/i915: Remove i915_gem_object_get_dirty_page()

Last user removed, remove the get_dirty_page convenience function.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200708173748.32734-4-chris@chris-wilson.co.uk