Previously the driver would always scale the chosen video mode to fill the
panel.  This adds 1:1 and maintain-aspect-ratio scaling modes.

v2: the drm_calloc/drm_free is replaced by kzalloc/kfree based
on Eric's suggestion.
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>