drm/i915: move i915_gem_vm_lookup() where it's used

Move the function next to the only user. Arguably it's perhaps not the
best place, but it's much better than having a static inline in a
header.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a080e401840a8b9d45946ff33fd63c7939a623ae.1644507885.git.jani.nikula@intel.com