drm/i915: add enable_ips module option

IPS is still enabled by default. Feature requested by the power
management team.

This should also help testing the feature on some early pre-production
hardware where there were relationship problems between IPS and PSR.

v2: Rebase on top of the newest IPS implementation.
v3: Check i915_enable_ips at compute_config, not supports_ips, so the
    kernel parameter will be ignored at haswell_get_pipe_config.

Requested-by: Kristen Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index a18da3c..be869f2 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1470,6 +1470,7 @@
 extern int i915_enable_ppgtt __read_mostly;
 extern unsigned int i915_preliminary_hw_support __read_mostly;
 extern int i915_disable_power_well __read_mostly;
+extern int i915_enable_ips __read_mostly;
 
 extern int i915_suspend(struct drm_device *dev, pm_message_t state);
 extern int i915_resume(struct drm_device *dev);