drm/i915: Enable PSR by default on Valleyview and Cherryview.

With a reliable frontbuffer tracking and all instability corner cases
solved for this platform let's re-enabled PSR by default.

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,
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>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_psr.c

@@ -780,7 +780,10 @@ void intel_psr_init(struct drm_device *dev)
 
 	/* Per platform default */
 	if (i915.enable_psr == -1) {
-		i915.enable_psr = 0;
+		if (IS_VALLEYVIEW(dev) || IS_CHERRYVIEW(dev))
+			i915.enable_psr = 1;
+		else
+			i915.enable_psr = 0;
 	}
 
 	/* Set link_standby x link_off defaults */