drm/i915: Mask LPSP to get PSR working even with Power Well in use by audio.

Power Well in use forces constantly PSR to exit.
On recent Kernel I noticed that PSR Performance Counter was always 0
indicating that PSR was never really achieved.
By masking LPSP, PSR can work normally and save power on Haswell.

Two bugs had been raised with PSR flag enabled:
- "Screen flickers when booted by enabling PSR in the kernel (i915.enable_psr=1) , the system is booting to a gray screen."
- "When booting the DUT with PSR feature enabled in the kernel (i915.enable_psr=1) , the system is booting to a gray screen."

Both bugs has been fixed by this patch.

v2: proper comment for -fixes

Tested-by: Selvaraj, Elavarasan <elavarasanx.selvaraj@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -1467,7 +1467,7 @@ static void intel_edp_psr_setup(struct intel_dp *intel_dp)
 
 	/* Avoid continuous PSR exit by masking memup and hpd */
 	I915_WRITE(EDP_PSR_DEBUG_CTL, EDP_PSR_DEBUG_MASK_MEMUP |
-		   EDP_PSR_DEBUG_MASK_HPD);
+		   EDP_PSR_DEBUG_MASK_HPD | EDP_PSR_DEBUG_MASK_LPSP);
 
 	intel_dp->psr_setup_done = true;
 }