drm/i915: Enable PSR by default on Haswell and Broadwell.

With a reliable frontbuffer tracking and all instability corner cases
on Haswell and Broadwell solved let's re-enabled PSR by default on
these platforms.

In case a new issue is found and PSR is the main suspect, please check
if i915.enable_psr=0 really makes your problem go away. If this is the case
PSR is the culprit so after that please check if i915.enable_psr=2
or i915.enable_psr=3 solves your issue and please let us know.
There are many panels out there and not all implementations apparently
work as we would expect.

In case you needed to force it on standby or disabled or in case of any
PSR related bug please report it at bugs.freedesktop.org.
In a bugzilla entry for PSR is desirable:
- dmesg (drm.debug=0xe)
- output of /sys/kernel/debug/dri/0/i915_edp_psr_status
- Platform information. Vendor, model, id, pci id.
- Graphical environment: Gnome, KDE, openbox, etc...
- Details how to reproduce.
- Also good if you could run PSR test cases of Intel-gpu-tools
- Please mention if forcing main link standby or main link off helps you.

There are Intel-gpu-tools test cases that can be helpful to
determine if PSR is working as expected:
kms_psr_sink_crc and kms_psr_frontbuffer_tracking.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455278893-1307-2-git-send-email-rodrigo.vivi@intel.com


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