drm/i915: Hook PSR functionality

PSR must be enabled after transcoder and port are running.
And it is only available for HSW.

v2: move enable/disable to intel_ddi
v3: The spec suggests PSR should be disabled even before backlight (by pzanoni)
v4: also disabling and enabling whenever panel is disabled/enabled.
v5: make it last patch to avoid breaking whenever bisecting. So calling for
    update and force exit came to this patch along with enable/disable calls.
v6: Remove unused and unecessary psr_enable/disable calls, as notice by Paulo.

CC: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Drop the psr exit code in the busy ioctl since I didn't merge
that part of the infrastructure yet - it needs more thought.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>