drm/i915/skl: disable display side power well support for now

The display power well support on this platform is in a somewhat broken
state atm, so disable it by default.

This in effect will get rid of incorrect assert WARNs about the CSR/DMC
firmware not being loaded during power well toggling. It also removes a
problem during driver loading where a register is accessed while its
backing power well is down, resulting in another WARN. Until we come up
with the root cause of the second problem and the proper fix for both
issues, keep all display side power wells on.

Also clarify a bit the option description.

Reported-by: Dave Airlie <airlied@redhat.com>
Reference: http://mid.gmane.org/CAPM=9tyjBQjSBTKa49cRr6SYkpNW7Pq-fUFznZZ8Y1snvvk7mA@mail.gmail.com
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446757451-2777-1-git-send-email-imre.deak@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_params.c

@@ -38,7 +38,7 @@ struct i915_params i915 __read_mostly = {
 	.enable_ppgtt = -1,
 	.enable_psr = 0,
 	.preliminary_hw_support = IS_ENABLED(CONFIG_DRM_I915_PRELIMINARY_HW_SUPPORT),
-	.disable_power_well = 1,
+	.disable_power_well = -1,
 	.enable_ips = 1,
 	.prefault_disable = 0,
 	.load_detect_test = 0,
@@ -127,7 +127,8 @@ MODULE_PARM_DESC(preliminary_hw_support,
 
 module_param_named_unsafe(disable_power_well, i915.disable_power_well, int, 0600);
 MODULE_PARM_DESC(disable_power_well,
-	"Disable the power well when possible (default: true)");
+	"Disable display power wells when possible "
+	"(-1=auto [default], 0=power wells always on, 1=power wells disabled when possible)");
 
 module_param_named_unsafe(enable_ips, i915.enable_ips, int, 0600);
 MODULE_PARM_DESC(enable_ips, "Enable IPS (default: true)");