drm/amdgpu/sriov:increate mailbox polling timeout

increase timeout to 12 seconds,because there may have multiple
FLR waiting for done, the waiting time of events may be long,
increase to 12s to reduce timeout failure.

Signed-off-by: Horace Chen <horace.chen@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>