drm/amdgpu: Use iterator methods exposed by amdgpu_res_cursor.h in building SG_TABLE's for a VRAM BO

Extend current implementation of SG_TABLE construction method to
allow exportation of sub-buffers of a VRAM BO. This capability will
enable logical partitioning of a VRAM BO into multiple non-overlapping
sub-buffers. One example of this use case is to partition a VRAM BO
into two sub-buffers, one for SRC and another for DST.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>