drm/i915/skl: Adding DDI_E power well domain

From B spec, DDI_E port belong to PowerWell 2, but
DDI_E share the powerwell_req/staus register bit with
DDI_A which belong to DDI_A_E_POWER_WELL.

In order to communicate with the connector on DDI-E, both
DDI_A_E_POWER_WELL and POWER_WELL_2 must be enabled.

Currently intel_dp_power_get(DDI_E) only enable
DDI_A_E_POWER_WELL, this patch will not only enable
DDI_a_E_POWER_WELL but also enable POWER_WELL_2.

This patch also fix the DDI-E hotplug function.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>