Similarly to what is done for SKL, clear the dpll_hw_state of the pipe
config in hsw_dp_set_ddi_pll_sel(), since it main contain stale values.
That can happen if a crtc that was previously driving an HDMI connector
switches to a DP connector. In that case, the wrpll field was left with
its old value, leading to warnings like the one below:

[drm:check_crtc_state [i915]] *ERROR* mismatch in dpll_hw_state.wrpll (expected 0xb035061f, found 0x00000000)
------------[ cut here ]------------
WARNING: CPU: 1 PID: 767 at drivers/gpu/drm/i915/intel_display.c:12324 check_crtc_state+0x975/0x10b0 [i915]()
pipe state doesn't match!

This regression was indroduced in

commit dd3cd74a
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Fri May 15 13:34:29 2015 +0300

    drm/i915: Don't overwrite (e)DP PLL selection on SKL
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>