drm/i915/tv: add ->get_config callback

We need this to properly fill in adjusted_mode.crtc_clock, otherwise
the state checker gets unhappy. This seems to have been forgotten in
the big clock rework in

commit 18442d08
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Sep 13 16:00:08 2013 +0300

    drm/i915: Fix port_clock and adjusted_mode.clock readout all over

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>