drm/i915: Enable AUX power earlier

For DDI/TypeC ports the AUX power domain needs to be enabled before the
port's PLL is enabled, so move the enabling earlier accordingly.

v2:
- Preserve the pre_pll hook for GEN9_LP. (Ville)
v3:
- Add related BSpec entries to commit log. (Jose)
v4:
- Rebase on the upstream ICL pre_pll_enable change.

BSpec: 21750, 22243
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101140427.31026-6-imre.deak@intel.com