Interrupt identity register we already read from hardware
contains engine class and instance fields. Leverage
these fields to find correct engine to handle the interrupt.

v3: rebase on top of rps intr
    use correct class / instance limits (Michel)
v4: split engine/other handling
v5: empty iir is not err (Daniele, Michel)
Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180406093145.14389-1-mika.kuoppala@linux.intel.com