Revert "Documentation/gpu/meson: Remove link to meson_canvas.c"

This reverts commit a3f98bb2.

Patch "Documentation/gpu/meson: Remove link to meson_canvas.c" was
incorrectly applied on the wrong branch not containing the fixed
commit 2bf6b5b0

 ("drm/meson: exclusively use the canvas provider module")

Acked-by: Sean Paul <sean@poorly.run>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190404144342.15238-1-narmstrong@baylibre.com