drm/vkms: refactor overlay plane creation

Move the logic to create an overlay plane to its own function.
Refactor, no functional changes.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220107182809.141003-1-jose.exposito89@gmail.com