drm: kernel-doc: add description for a new function parameter

drm: kernel-doc: add description for a new function parameter

As reported by "make htmldocs":

	./drivers/gpu/drm/drm_prime.c:808: warning: Function parameter or member 'dev' not described in 'drm_prime_pages_to_sg'

Add a description for the new parameter.

Fixes: 707d561f77b5 ("drm: allow limiting the scatter list size.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/9366f48e6e9c3ec2f31a3e68452a2b23a1089fce.1603791716.git.mchehab+huawei@kernel.org