kmssink: add a plane-id property

kmssink: add a plane-id property

Without setting the DRM_CLIENT_CAP_UNIVERSAL_PLANES capability bit, only
overlay planes are made available for compatibility with legacy clients.

But if a CRTC doesn't have an overlay plane associated, then kmssink is
not able to find a plane for the CRTC and the pipeline will fail, i.e:

ERROR                kmssink gstkmssink.c:482:gst_kms_sink_start:<kmssink0> Could not find a plane for crtc

This patch adds a plane-id property to the kmssink element so a specific
plane can be used in case that a CRTC has only a primary plane associated.

https://bugzilla.gnome.org/show_bug.cgi?id=768183