drm/i915/display: Reset message bus after each read/write operation

Every know and then we receive the following error when running
for example IGT test kms_flip.

[drm] *ERROR* PHY G Read 0d80 failed after 3 retries.
[drm] *ERROR* PHY G Write 0d81 failed after 3 retries.

Since the error is sporadic in nature, the patch proposes
to reset the message bus after every successful or unsuccessful
read or write operation.

v2: Add FIXME's to indicate the experimental nature of
    this workaround (Rodrigo)
v3: Dropping the additional delay as moving reset to *_read_once()
    and *_write_once() functions seem unnecessary delay
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231016125544.719963-1-mika.kahola@intel.com

drivers/gpu/drm/i915/display/intel_cx0_phy.c

@@ -206,6 +206,13 @@ static int __intel_cx0_read_once(struct drm_i915_private *i915, enum port port,
 
 	intel_clear_response_ready_flag(i915, port, lane);
 
+	/*
+	 * FIXME: Workaround to let HW to settle
+	 * down and let the message bus to end up
+	 * in a known state
+	 */
+	intel_cx0_bus_reset(i915, port, lane);
+
 	return REG_FIELD_GET(XELPDP_PORT_P2M_DATA_MASK, val);
 }
 
@@ -285,6 +292,13 @@ static int __intel_cx0_write_once(struct drm_i915_private *i915, enum port port,
 
 	intel_clear_response_ready_flag(i915, port, lane);
 
+	/*
+	 * FIXME: Workaround to let HW to settle
+	 * down and let the message bus to end up
+	 * in a known state
+	 */
+	intel_cx0_bus_reset(i915, port, lane);
+
 	return 0;
 }