drm/i915/mtl: Disable MC6 for MTL A step

The Wa_14017073508 require to send Media Busy/Idle mailbox while
accessing Media tile. As of now it is getting handled while __gt_unpark,
__gt_park. But there are various corner cases where forcewakes are taken
without __gt_unpark i.e. without sending Busy Mailbox especially during
register reads. Forcewakes are taken without busy mailbox leads to
GPU HANG. So bringing mailbox calls under forcewake calls are no feasible
option as forcewake calls are atomic and mailbox calls are blocking.
The issue already fixed in B step so disabling MC6 on A step and
reverting previous commit which handles Wa_14017073508

Fixes: 8f70f1ec

 ("drm/i915/mtl: Add Wa_14017073508 for SAMedia")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230310061339.2495416-2-badal.nilawar@intel.com