drm/msm/gem: Rename internal get_iova_locked helper

We'll need to introduce a _locked() version of msm_gem_get_iova(), so we
need to make that name available.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>