drm/amdgpu:set cond_exec polling value to 1 in ring_init

no need to set it per ib_schedule(), hw won't override
this polling address.

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