[media] smiapp: Provide a common function to obtain native pixel array size

The same pixel array size is required for the active format of each
sub-device sink pad and try format of each sink pad of each opened file
handle as well as for the native size rectangle.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>