drm/i915: Remove spurious DRM_ERROR for cancelled interrupts

As we ourselves cancel interrupts during reset by clearing the GTIIR, it
is possible for the master IIR to indicate a pending IRQ for which we
have already cleared from the GTIIR. In this case, the DRM_ERROR are
intended and should not be flagged as an error.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180202153448.23908-1-chris@chris-wilson.co.uk