r600: don't do stack workarounds for hemlock

By the looks of it it seems hemlock is treated separately to cypress, but
certainly it won't need the stack workarounds cedar/redwood (and
seemingly every other eg chip except cypress/juniper) need.
(Discovered by accident.)

Acked-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit c2f0e088)