drm/i915/execlists: Split spinlock from its irq disabling side-effect

drm/i915/execlists: Split spinlock from its irq disabling side-effect

During reset/wedging, we have to clean up the requests on the timeline
and flush the pending interrupt state. Currently, we are abusing the irq
disabling of the timeline spinlock to protect the irq state in
conjunction to the engine's timeline requests, but this is accidental
and conflates the spinlock with the irq state. A baffling state of
affairs for the reader.

Instead, explicitly disable irqs over the critical section, and separate
modifying the irq state from the timeline's requests.

Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302143246.2579-4-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>