drm/i915: Fix reason for per-chip disabling of FBC

When running on my snb machine, recent kernels display successively:

[drm:intel_update_fbc], fbc set to per-chip default
[drm:intel_update_fbc], fbc disabled per module param

But no module param is set. This happens because the check for the
module parameter uses a variable that has been overridden inside the
"per-chip default" code.

Fix up the logic and add another reason for the FBC to the be disabled.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>