drm/rockchip: vop2: initialize possible_crtcs properly

The variable possible_crtcs is only initialized for primary and
overlay planes. Since the VOP2 driver only supports these plane
types at the moment, the current code is safe. However, in order
to provide a future-proof solution, fix the initialization of
the variable.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230124054706.3921383-2-michael.riesch@wolfvision.net