drm/vkms: Remove index parameter from init_vkms_output

drm/vkms: Remove index parameter from init_vkms_output

VKMS currently supports only one CRTC, so it make no sense to have this
index configurable. To avoid issues, replace this hardcoded index by
drm_crtc_mask when applicable.

There is no need to manually set a crtc mask on primary and cursor plane
as it is automatically set by drmm_crtc_alloc_with_planes.

In addition, this will remove the use of an uninitialized structure in
vkms_add_overlay_plane. This currently works by chance because two things:
- vkms_plane_init always set a possible_crtcs value, so the problematic
  branch is never used;
- drm_crtc_mask on an kzalloc'd drm_crtc returns BIT(0), and the VKMS CRTC
  always have this id.

Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241119-vkms-remove-index-v3-1-976321a3f801@bootlin.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>