amdgpu: add IB sharing support v2

A new 'offset' flag is added to the amdgpu_cs_ib_info structure,
which can be used to specify location of PM4 packets to execute
in the IB buffer object

v2: remove the shared IB flag, etc

Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>