drm/i915: set up PIPECONF explicitly for i9xx/vlv platforms

Same reasons as for the previous patch, just no bug report about
anything going wrong yet: We only support exactly the mode we program,
so don't leave any stale BIOS state behind.

Again this will be fun to properly track for fastboot.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>