drm: writeback: Add client capability for exposing writeback connectors

drm: writeback: Add client capability for exposing writeback connectors

Due to the fact that writeback connectors behave in a special way
in DRM (they always report being disconnected) we might confuse some
userspace. Add a client capability for writeback connectors that will
filter them out for clients that don't understand the capability.

Changelog:
 - only accept the capability if the client has already set the
DRM_CLIENT_CAP_ATOMIC one.

Cc: Sean Paul <[email protected]>
Cc: Brian Starkey <[email protected]>
Signed-off-by: Liviu Dudau <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>
Reviewed-by: Brian Starkey <[email protected]>
Link: https://patchwork.freedesktop.org/patch/229038/