drm/ttm: Add helper functions to populate/map in one call (v2)

These functions replace a section of common code found
in radeon/amdgpu drivers (and possibly others) as part
of the ttm_tt_*populate() callbacks.

v2: squash in fix for sw iommu from Tom

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>