gpu: ion: Leave fmem in c-state until unsecured

When the last buffer is freed from a heap that uses fmem we call
into fmem to transition the memory from c-state to t-state. However,
if the heap has been secured for content protection transitioning
the memory from c-state to t-state could cause undefined behavior
in fmem.

Add checks to ensure that a secured heap will not transition to
t-state until heap is unsecured.