drm/i915/mtl: Disable C6 on MTL A0 for media

Earlier merge dropped an if block when applying the patch -
"drm/i915/mtl: Synchronize i915/BIOS on C6 enabling". Bring back the
if block as the check is required by - "drm/i915/mtl: Disable MC6 for MTL
A step" to disable C6 on media for A0 stepping.

Fixes: 37350409 ("drm/i915/mtl: Synchronize i915/BIOS on C6 enabling")
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230324213918.75212-1-umesh.nerlige.ramappa@intel.com