drm/radeon: re-enable PTE/PDE packet for set_page on cayman/TN

PTE/PDE doesn't support a single update (count = 1).  We had
previously disabled it since it we were hitting that case which
let to hangs.  The PTE/PDE packet is much more efficient for VM
updates where it can be used.

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