drm/i915: Enable PSR for Baytrail and Braswell.

This patch is the last in series of VLV/CHV PSR,
that finally enable PSR by adding it to HAS_PSR
and calling the proper enable and disable
functions on the right places.

Although it is still disabled by default.

v2: Rebase over intel_psr and merge Durgadoss's fixes.
v3: Fix typo.

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