drm/meson: Convert existing documentation to actual kerneldoc

drivers/gpu/drm/meson/meson_canvas.c

drivers/gpu/drm/meson/meson_drv.c

drivers/gpu/drm/meson/meson_dw_hdmi.c

drivers/gpu/drm/meson/meson_vclk.c

drivers/gpu/drm/meson/meson_venc.c

drivers/gpu/drm/meson/meson_viu.c

drivers/gpu/drm/meson/meson_vpp.c