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: 2bf6b5b0e374 ("drm/meson: exclusively use the canvas provider module")
Cc: Maxime Jourdan <[e-mail]>
Cc: Neil Armstrong <[e-mail]>
Cc: Kevin Hilman <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Acked-by: Neil Armstrong <[e-mail]>
Signed-off-by: Sean Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/meson.rst

···
 .. 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
 ============