drm: Add plane->name and use it in debug prints

Show a sensible name for the plane in debug mesages. The driver
may supply its own name, otherwise the core genrates the name
("plane-0", "plane-1" etc.).

v2: kstrdup() the name passed by the caller (Jani)
v3: Generate a default name if the driver doesn't supply one

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449592922-5545-7-git-send-email-ville.syrjala@linux.intel.com