drm: clarify scaling property names

Now that we're using the scaling property in the Intel driver I noticed
that the names were a bit confusing.  I've corrected them according to
our discussion on IRC and the mailing list, though I've left out
potential new additions for a new scaling property with an integer (or
two) for the scaling factor.  None of the drivers implement that today,
but if someone wants to do it, I think it could be done with the
addition of a single new type and a new property to describe the
scaling factor in the X and Y directions.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>