drm/i915/guc: unify guc irq handling

The 16-bit guc irq vector is unchanged across gens, the only thing that
moved is its position (from the upper 16 bits of the PM regs to its own
register). Instead of duplicating all defines and functions to handle
the 2 different positions, we can work on the vector and shift it as
appropriate. While at it, update the handler to work on intel_guc.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190713100016.8026-5-chris@chris-wilson.co.uk
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>