drm/i915: Enable PSR by default.

Panel Self Refresh is an eDP power saving feature specified by VESA's eDP v1.3,
that allows some panel componets to shutdown while you still see static images on
the screen. Besides being supported on the platform it must be supported by the
eDP panel itself.

Now that we have the propper frontbuffer tracking support and correct locks on place
we can enabled this feature by default.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>