drm: mali-dp: Report writeback connector as connected

drm: mali-dp: Report writeback connector as connected

Older version of this patch series reported writeback as disconnected
to avoid confusing userspace not aware of writeback connectors.
However, the version that got merged uses a special cap
(DRM_CLIENT_CAP_WRITEBACK_CONNECTORS) for this purpose.

This helps us avoid some special handling of writeback connector
in drm_helper_probe_single_connector_modes, see [1].

https://lists.freedesktop.org/archives/dri-devel/2018-July/183144.html

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>