drm/i915: Fix PSR2 selective update corruption after PSR1 setup

There is probably a issue in DMC firmwares(icl_dmc_ver1_07.bin and
kbl_dmc_ver1_04.bin at least) that causes PSR2 SU to fail after
exiting DC6 if EDP_PSR_TP1_TP3_SEL is kept in PSR_CTL, so for now
lets workaround the issue by cleaning PSR_CTL before enable PSR2.

v2:
- Updated commit description and comment to state that it may be
a DMC firmware issue (Rodrigo)
- No need to RMW, let's write 0 to PSR_CTL(Dhinakaran)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190314230113.6571-1-jose.souza@intel.com

drivers/gpu/drm/i915/intel_psr.c

@@ -530,6 +530,14 @@ static void hsw_activate_psr2(struct intel_dp *intel_dp)
 	else
 		val |= EDP_PSR2_TP2_TIME_2500us;
 
+	/*
+	 * FIXME: There is probably a issue in DMC firmwares(icl_dmc_ver1_07.bin
+	 * and kbl_dmc_ver1_04.bin at least) that causes PSR2 SU to fail after
+	 * exiting DC6 if EDP_PSR_TP1_TP3_SEL is kept in PSR_CTL, so for now
+	 * lets workaround the issue by cleaning PSR_CTL before enable PSR2.
+	 */
+	I915_WRITE(EDP_PSR_CTL, 0);
+
 	I915_WRITE(EDP_PSR2_CTL, val);
 }