drm/i915/hwmon: Silence "mailbox access failed" warning in snb_pcode_read

hwm_pcode_read_i1 is called during i915 load. This results in the following
warning from snb_pcode_read because POWER_SETUP_SUBCOMMAND_READ_I1 is
unsupported on DG1/DG2.

[drm:snb_pcode_read [i915]] warning: pcode (read from mbox 47c) \
		mailbox access failed for snb_pcode_read_p [i915]: -6

The code handles the unsupported command but the warning in dmesg is
a red herring which has resulted in a couple of bugs being filed.
Therefore silence the warning by avoiding calling snb_pcode_read_p
for DG1/DG2.
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221203031454.1280538-1-ashutosh.dixit@intel.com

drivers/gpu/drm/i915/i915_hwmon.c

@@ -293,6 +293,10 @@ static const struct hwmon_channel_info *hwm_gt_info[] = {
 /* I1 is exposed as power_crit or as curr_crit depending on bit 31 */
 static int hwm_pcode_read_i1(struct drm_i915_private *i915, u32 *uval)
 {
+	/* Avoid ILLEGAL_SUBCOMMAND "mailbox access failed" warning in snb_pcode_read */
+	if (IS_DG1(i915) || IS_DG2(i915))
+		return -ENXIO;
+
 	return snb_pcode_read_p(&i915->uncore, PCODE_POWER_SETUP,
 				POWER_SETUP_SUBCOMMAND_READ_I1, 0, uval);
 }