drm/i915: make sure eDP PLL is enabled at the right time

We need to make sure the eDP PLL is enabled before the pipes or planes,
so do it as part of the DP prepare mode set function.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

diff --cc drivers/gpu/drm/i915/intel_display.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_dp.c
Simple merge