drm/i915/bxt: Assign PLL for pipe

Assign PLL for pipe (dependent on port attached to the pipe)

v2:
- fix incorrect encoder vs. new_encoder check for crtc (imre)

v3:
- warn and return error if no encoder is attached (imre)

Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v2)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Don't move intel_ddi_get_crtc_new_encoder around.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>