drm/i915/fbc: Remove intel_fbc_global_disable()

By the time intel_fbc_global_disable() gets called during driver
teardown we should have already disabled all the crtcs, so
no way FBC should be enabled at this point.

And I have no idea what the other user (i915_restore_display())
is even trying to achieve.

So let's just throw intel_fbc_global_disable() into the bin.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315140001.1172-6-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>