drm/i915/mtl: Synchronize i915/BIOS on C6 enabling

If BIOS enables/disables C6, i915 should do the same. Also, retain
this value across driver reloads. This is needed only for MTL as
of now due to an existing bug in OA which needs C6 disabled for
it to function. BIOS behavior is also different across platforms
in terms of how C6 is enabled.

Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323225901.3743681-3-umesh.nerlige.ramappa@intel.com