drm/i915: kick firmware fbs even when i915 fbdev is disabled

Otherwise we don't kick out firmware framebuffers like vesafb and
efifb when CONFIG_DRM_I915_FBDEV=n but CONFIG_FB=y.

There's still the pesky issue with vgacon which we should somehow
replace with the dummy console at least. We have a similar issue at
module un/reload, since vgacon state is terminally botched after
i915.ko has loaded in modeset mode. But this gets us a step further at
least.

v2: Use IS_ENABLED - I always get this wrong for tristates. Spotted by
Jani.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>