drm/i915: encoder->get_config is no longer optional

We must have one to fill out the adjusted_mode.crtc_clock. And with
the tv encoder fixed up every encoder we have has a ->get_config
callback. So we can drop the checks.

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