mode: Retrieve only the current information for a Connector

Add a new API that allows the caller to skip any forced probing, which
may require slow i2c to a remote display, and only report the currently
active mode and encoder for a Connector. This is often the information
of interest and is much, much faster than re-retrieving the link status
and EDIDs, e.g. if the caller only wishes to count the number of active
outputs.

v2: Fix error path to avoid double free after a failed GETCONNECTOR
ioctl.

v3: Daniel strongly disapproved of my disjoint in behaviour between
GetConnector and GetConnectorCurrent, and considering how best to make a
drop in replacement for drmmode_output_init() convinced me keeping the
API as consistent as possible was the right approach.

v4: Avoid probing on the second calls to GETCONNECTOR for unconnected
outputs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: David Herrmann <dh.herrmann@googlemail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>