drm/i915: Check for error before calling cmpxchg()

Only do the locked compare of the existing fence->error if we actually
need to set an error. As we tend to call i915_sw_fence_set_error_once()
unconditionally, it saves on typing to put the common has-error check
into the inline.

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/20191206160428.1503343-1-chris@chris-wilson.co.uk