drm: Warn about negative sizes when calculating scale factor

Passing negative width/hight to scale factor calculations is not
legal. Let's WARN if that happens.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1469549224-1860-2-git-send-email-ville.syrjala@linux.intel.com