drm/i915/psr: Set DPCD PSR2 enable bit when needed

In the 2 eDP1.4a pannels tested set or not set bit have no effect
but is better set it and comply with specification.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328223046.16125-9-jose.souza@intel.com