drm-misc: Another check in the bugfix flow

Even in feature freeze it can happen that the broken patch is only in
drm-misc-next, and then applying to drm-misc-next-fixes will
rightfully stumble over dim's Fixes: validation since the sha1 wont be
an ancestor.

Fix that by adding another check.

v2: Less confusing wording (Marek)

Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
3 jobs for master in 38 seconds (queued for 43 seconds)