drm: remove excess description

Description of expected_size doesn't match any parameter of the function
drm_atomic_replace_property_blob. Removing it.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458408156-15990-1-git-send-email-luisbg@osg.samsung.com