genX_mfd: no need for loop filtering for skipped pictures

genX_mfd: no need for loop filtering for skipped pictures

Skipped pictures are equal to their reference frame and, therefore,
should not pass the loop filter. Although the loop filter was already
a null operation as all transform coefficients are zero for skipped
pictures, this may save some cyles in the decoder hardware.

Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>