drm/i915: stop killing pfit on i9xx

Nowadays (i.e. with Valleyview) we also have edp on non-PCH_SPLIT
platforms, so just checking for LVDS is not good enough.

Secondly we have full pfit pipe config tracking, so we'll correctly
disable the pfit as part of the initial modeset.

For fastboot we need a bit of work here to correctly kill unsupported
configs (if e.g. the pfit is used on anything else than the built-in
panel). But since that's not yet supported we don't need to worry.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>