DDS: fix image format check

Replace duplicated bBitMask check by the intended gBitMask check.

Change-Id: I6d1780a4112e88484de661f07a15f2121602b86c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>