drm/i915: Enable PSR2 by default

The support for PSR2 was polished, IGT tests for PSR2 was added and
it was tested performing regular user workloads like browsing,
editing documents and compiling Linux, so it is time to enable it by
default and enjoy even more power-savings.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190308000050.6226-9-jose.souza@intel.com