drm: zte: make zx_plane accessible from zx_vou driver

Move struct zx_plane from zx_plane.c to zx_plane.h, so that it can be
accessed from zx_vou driver, and we can save the use of struct
zx_layer_data completely.  More importantly, those additional data used
by VOU controller to enable/disable graphic and video layers can later
be added and accessed much more easily from zx_vou driver.

While at it, we make two changes to zx_plane_init() interface:

 - Encode struct device pointer in zx_plane, so that we do not need to
   pass it as a parameter.
 - Change return of zx_plane_init() from struct drm_plane pointer to
   error code, since we can get the pointer from zx_plane in zx_vou
   driver now.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>