drm/rockchip: vop2: fix uninitialized variable possible_crtcs

clang reportes this error
drivers/gpu/drm/rockchip/rockchip_drm_vop2.c:2322:8: error:
  variable 'possible_crtcs' is used uninitialized whenever 'if'
  condition is false [-Werror,-Wsometimes-uninitialized]
                        if (vp) {
                            ^~
drivers/gpu/drm/rockchip/rockchip_drm_vop2.c:2336:36: note:
  uninitialized use occurs here
                ret = vop2_plane_init(vop2, win, possible_crtcs);
                                                 ^~~~~~~~~~~~~~
drivers/gpu/drm/rockchip/rockchip_drm_vop2.c:2322:4:
  note: remove the 'if' if its condition is always true
                        if (vp) {
                        ^~~~~~~~

The else-statement changes the win->type to OVERLAY without setting the
possible_crtcs variable.  Rework the block, initialize possible_crtcs to
0 to remove the else-statement.  Split the else-if-statement out to its
own if-statement so the OVERLAY check will catch when the win-type has
been changed.

Fixes: 368419a2 ("drm/rockchip: vop2: initialize possible_crtcs properly")
Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316132302.531724-1-trix@redhat.com