drm/vkms: Unset preferred_depth

drm/vkms: Unset preferred_depth

There's a confusion between the preferred_depth uapi and the generic
fbdev helpers. Former wants depth, latter wants bpp, and for XRGB8888
they don't match. Which hit me with vkms, which wants that.

All other drivers setting this and using the generic fbdev helpers use
16, where both numbers match, for RGB565.

Since fixing this is a bit involved (I think for atomic drivers we
should just compute this all internally from the format list of the
first primary plane) paper over the issue in vkms by using defaults
everywhere. Then userspace will pick XRGB8888, and fbdev helpers will
do the same, and we have what we want.

Reported-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Simon Ser <contact@emersion.fr>
Cc: Simon Ser <contact@emersion.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Melissa Wen <melissa.srw@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211161113.3350061-1-daniel.vetter@ffwll.ch