drm/i915/fbc: Disable fbc by default on all glk+

drm/i915/fbc: Disable fbc by default on all glk+

We're missing a workaround in the fbc code for all glk+ platforms
which can cause corruption around the top of the screen. So
enabling fbc by default is a bad idea. I'm not keen to backport
the w/a so let's start by disabling fbc by default on all glk+.
We'll lift the restriction once the w/a is in place.

Cc: stable@vger.kernel.org
Cc: Daniel Drake <drake@endlessm.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jian-Hong Pan <jian-hong@endlessm.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-2-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
(cherry picked from commit cd8c021b36a66833cefe2c90a79a9e312a2a5690)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>