drm/i915: Nuke .post_pll_disable() for DDI platforms

HSW+ platforms call encoder .post_disable() and .post_pll_disable()
back to back. And since we don't even disable the PLL in between
let's just move everything into .post_disable().

intel_dp_mst does forward the .post_disable() call to intel_ddi at
the very end of its own .post_disable() hook, so this time MST
I shouldn't even break MST by accident.

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213195217.15168-2-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>