drm/i915: Add kerneldoc for intel_pipe_update_{start, end}

Note that a later patch will use these functions in some other file
and drop the static. Hence the kerneldoc looks appropriate.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Add comment that the functions will become non-static
shortly.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>