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