habanalabs: remove compute context pointer

It was an error to save the compute context's pointer in the device
structure, as it allowed its use without proper ref-cnt.

Change the variable to a flag that only indicates whether there is
an active compute context. Code that needs the pointer will now
be forced to use proper internal APIs to get the pointer.

Signed-off-by: Oded Gabbay <ogabbay@kernel.org>