drm/i915: Add an i915_gem_vm_lookup helper

This is the VM equivalent of i915_gem_context_lookup.  It's only used
once in this patch but future patches will need to duplicate this lookup
code so it's better to have it in a helper.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-20-jason@jlekstrand.net