drm/i915: Remove some dead struct fwd decl from i915_drv.h

Gone with userptr rewrite by Maarten in ed29c269 ("drm/i915: Fix
userptr so we do not have to worry about obj->mm.lock, v7.")

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211022082200.2684194-1-daniel.vetter@ffwll.ch