Because we're currently using FBC_UNSUPPORTED_MODE for two different
cases.

This commit will also allow us to write the next one without hiding
information from the user.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>