drm/meson: store the framebuffer width for plane commit

Also store the framebuffer width in the private common struct
to be used by the AFBC decoder module driver when committing the AFBC
plane.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021091509.3864-4-narmstrong@baylibre.com