drm/i915: split encoder get_config calls from crtc get_clock calls

This should help on HSW, where we don't currently have a get_clock call.

Reported-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>