[drm] Store mode of each head as index

[drm] Store mode of each head as index

Previously, we would store the mode of each head as pointer in
the array of modes on the connector object.  Now we just store
the index into that array.

This is to make it clearer that we don't own the memory
associated with it directly, and that the memory associated with
it is automatically cleaned up when the connector is.

This also helps to highlight a problem mentioned by
Forest Bond <forest@alittletooquiet.net> here:

http://lists.freedesktop.org/archives/plymouth/2010-June/000369.html

Namely, we've been naïvely treating the first available mode in
the connector object as the active mode.  While this is true
most of the time, it doesn't hold true if the user overrides the
mode on the kernel command line.