drm/i915/fbc: Introduce intel_fbc_sanitize()

Let's make sure FBC is always disabled when we start to take
over the hardware state.

I suspect this should never really happen, since the only time
when we really should be taking over with the display already
active is when the previous state was progammed by the BIOS,
which likely shouldn't use FBC. This could be driver init,
or S4 resume when the boot kernel doesn't load i915. But I
suppose no harm in keeping this code around for exra safety
since it's quite trivial.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315140001.1172-7-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>