drm/i915: change default panel fitting mode to preserve aspect ratio

We did this a long time ago in the DDX driver, but now this fix belongs
in the kernel.

Preserving the aspect ratio is a nicer default.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=18033

.

Tested-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>