drm/i915: Apply the PPS register unlock workaround more consistently

Atm, we apply this workaround somewhat inconsistently at the following
points: driver loading, LVDS init, eDP PPS init, system resume. As this
workaround also affects registers other than PPS (timing, PLL) a more
consistent way is to apply it early after the PPS HW context is known to
be lost: driver loading, system resume and on VLV/CHV/BXT when turning
on power domains.

This is needed by the next patch that removes saving/restoring of the
PP_CONTROL register.

This also removes the incorrect programming of the workaround on HSW+
PCH platforms which don't have the register locking mechanism.

v2: (Ville)
- Don't apply the workaround on BXT.
- Simplify platform checks using HAS_DDI().
v3:
- Move the call of intel_pps_unlock_regs_wa() to the more
  logical vlv_display_power_well_init() (also fixing CHV) (Ville).

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/1470827254-21954-5-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/intel_lvds.c

@@ -978,14 +978,6 @@ void intel_lvds_init(struct drm_device *dev)
 	int pipe;
 	u8 pin;
 
-	/*
-	 * Unlock registers and just leave them unlocked. Do this before
-	 * checking quirk lists to avoid bogus WARNINGs.
-	 */
-	if (HAS_PCH_SPLIT(dev_priv) || INTEL_GEN(dev_priv) <= 4)
-		I915_WRITE(PP_CONTROL(0),
-			   I915_READ(PP_CONTROL(0)) | PANEL_UNLOCK_REGS);
-
 	if (!intel_lvds_supported(dev))
 		return;