drm/vkms: Avoid assigning 0 for possible_crtc

drm/vkms: Avoid assigning 0 for possible_crtc

When vkms invoke drm_universal_plane_init(), it sets 0 for
possible_crtcs parameter which means that planes can't be attached to
any CRTC. It currently works due to some safeguard in the drm_crtc file;
however, it is possible to identify the problem by trying to append a
second connector. This patch fixes this issue by modifying
vkms_plane_init() to accept an index parameter which makes the code a
little bit more flexible and avoid set zero to possible_crtcs.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/d67849c62a8d8ace1a0af455998b588798a4c45f.1561491964.git.rodrigosiqueiramelo@gmail.com