In commit 38d5487d, Keith explained:
    This patch simply merges the two mode type bits together; that seems
    reasonable to me, but perhaps only a subset of the bits should be
    used? None of these can be user defined as they all come from
    looking at just the hardware.

However, merging the bits means that a flag becomes sticky. It is not
possible, for example to update the mode type to remove the
DRM_MODE_TYPE_PREFERRED bit.

After a brief discussion with Dave Airlie on irc, it was agreed to
propose that change, instead of introducing another function to remove a
bit from exisiting modes type.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>