drm/i915: Rearrange the comments in i915_add_request()

The comments were a little out-of-sequence with the code, forcing the
reader to jump around whilst reading. Whilst moving the comments around,
add one to explain the context reference.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>