drm/i915: Don't overwrite (e)DP PLL selection on SKL

In the following commit, the place where the contents of dpll_hw_state
in crtc_state where zeroed was changed. Prior to that commit, it
happened when the new state was allocated, but now that happens just
before the call the .crtc_compute_clock() hook. The DP code for SKL,
however, sets up the (private) PLL in the encoder compute config
function that has already run by the time that memset() is reached,
causing the previous value to be lost.

This patch fixes the issue by moving the memset() down the call chain,
so that it is only called if the values in dpll_hw_state are going to be
updated.

commit 4978cc93
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Tue Apr 21 17:13:21 2015 +0300

    drm/i915: Preserve shared DPLL information in new pipe_config

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90462


Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reported-and-tested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>