Planes are a bit like half-CRTCs.  They have a location and fb, but
don't drive outputs directly.  Add support for handling them to the core
KMS code.

v2: fix ABI of get_plane - move format_type_ptr to the end
v3: add 'flags' field for interlaced support (from Ville)
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Reviewed-by: Rob Clark <rob.clark@linaro.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>