drm/Documentation: Fix rowspan value in drm-kms-properties

The "DRM" rowspan wasn't updated in commit cc7096fb6d1d (drm/mode: document path
property and function to set it. (v1.1)), so increment it by one to fix the
table.

Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>