drm/i915/icl: Use hw engine class, instance to find irq handler

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