drm/i915: Fix a buch of kerneldoc warnings

Just a bunch of stale kerneldocs generating warnings when
building the docs. Mostly function parameters so not very
useful but still.

v2: Tidy.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464958937-23344-1-git-send-email-tvrtko.ursulin@linux.intel.com