drm/i915/guc: keep breadcrumb irq always enabled

drm/i915/guc: keep breadcrumb irq always enabled

We rely on the tasklet to update the GT PM refcount, so we can't disable
it even if we've processed all the requests for the engine because we
might have detected the request completion before the interrupt arrived.

Since on all platforms on which we plan to support guc submission we
don't allow disabling the breadcrumb interrupts, we can further siplify
the park/unpark flow by removing the interrupt pin/unpin. A BUG_ON has
been added to catch changes to this flow that would require us to
restore some kind of pinning.

v2: split removal of engine_pin/unpin_breadcrumbs_irq to its own
    patch (chris)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812233152.2172-1-daniele.ceraolospurio@intel.com