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 <[email protected]>
Cc: Sebastian Reichel <[email protected]>
Cc: Tomi Valkeinen <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: Gerd Hoffmann <[email protected]>
Cc: David Lechner <[email protected]>
Cc: Noralf Trønnes <[email protected]>
Cc: Hans de Goede <[email protected]>
Reviewed-by: Noralf Trønnes <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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