gma500: Make crtc count a property of the device

Octavian Purdila posted a patch that sets num_crtc to 1 for Moorestown, but
Oaktrail has 2 so we need to split Oaktrail/Moorestown more sensibly, and
also cope with some other differences later on.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>