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

The file was removed in the below patch and is causing this error:
WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -function Canvas ../drivers/gpu/drm/meson/meson_canvas.c' failed with return code

Fixes: 2bf6b5b0

 ("drm/meson: exclusively use the canvas provider module")
Cc: Maxime Jourdan <mjourdan@baylibre.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-amlogic@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190403205652.183496-1-sean@poorly.run