drm/msm: add internal MSM_BO_MAP_PRIV flag

This flag sets IOMMU_PRIV, which is required for some a6xx GMU objects.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Jordan Crouse <jcrouse@codeauorora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>