drm/i915/gt: Fix up missing error propagation for heartbeat pulses

Just missed setting err along an interruptible error path for the
intel_engine_pulse().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200218162150.1300405-4-chris@chris-wilson.co.uk