media: venus: don't de-reference NULL pointers at IRQ time

Smatch is warning that:
	drivers/media/platform/qcom/venus/hfi_venus.c:1100 venus_isr() warn: variable dereferenced before check 'hdev' (see line 1097)

The logic basically does:
	hdev = to_hfi_priv(core);

with is translated to:
	hdev = core->priv;

If the IRQ code can receive a NULL pointer for hdev, there's
a bug there, as it will first try to de-reference the pointer,
and then check if it is null.

After looking at the code, it seems that this indeed can happen:
Basically, the venus IRQ thread is started with:
	devm_request_threaded_irq()
So, it will only be freed after the driver unbinds.

In order to prevent the IRQ code to work with freed data,
the logic at venus_hfi_destroy() sets core->priv to NULL,
which would make the IRQ code to ignore any pending IRQs.

There is, however a race condition, as core->priv is set
to NULL only after being freed. So, we need also to move the
core->priv = NULL to happen earlier.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>