drm/omap: Move buffer pitch/offset to drm_framebuffer

drm_framebuffer already holds per-plane pitch and offsets, which is
filled out for us when we create the framebuffer. Nuke our local copy in
the plane struct.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180330141138.28987-7-daniels@collabora.com