drm/msm: Fix fb plane offset calculation

The offset got dropped by accident.

Fixes: d413e6f9

 ("drm/msm: Drop msm_gem_iova()")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Stephen Boyd <swboyd@chromium.org> # CoachZ
Link: https://lore.kernel.org/r/20220510165216.3577068-1-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>