drm/radeon: update ib_execute for SI (v2)

When submitting a CONST_IB, emit a SWITCH_BUFFER
packet before the CONST_IB.  This isn't strictly necessary
(the driver will work fine without it), but is good practice
and allows for more flexible DE/CE sychronization options
in the future.  Current userspace drivers do not take
advantage of the CE yet.

v2: - clean up code flow a bit
    - no need to flush caches for CONST IB

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