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: 2bf6b5b ("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

Documentation/gpu/meson.rst

@@ -42,12 +42,6 @@ Video Encoder
 .. kernel-doc:: drivers/gpu/drm/meson/meson_venc.c
    :doc: Video Encoder
 
-Video Canvas Management
-=======================
-
-.. kernel-doc:: drivers/gpu/drm/meson/meson_canvas.c
-   :doc: Canvas
-
 Video Clocks
 ============