drm/edid: don't return stack garbage from supports_rb

commit b196a4980ff7bb54db478e2a408dc8b12be15304 upstream.

We need to initialize this to false, because the is_rb callback only
ever sets it to true.

Noticed while reading through the code.

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>