intel: Fix documentation for drm_intel_gem_bo_wait()

The kernel doesn't actually wait indefinately when passed a negative,
timeout, it returns immediately.  Document this and suggest using INT64_MAX
for indefinite waits.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>