drm/fb: document dirty helper better

Apparently little known fact that there's no need to hand-roll your own
anymore. Cc'ing a bunch of driver people who might want to know this
too.

v2: s/none/known/ (Chris Wilson)

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sebastian Reichel <sebastian.reichel@collabora.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: David Lechner <david@lechnology.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611112859.16375-1-daniel.vetter@ffwll.ch