Remove (incorrect) filenames from x264 asm files, add descriptions.

Originally committed as revision 17523 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/x86/h264_deblock_sse2.asm

 ;*****************************************************************************
-;* deblock-a.asm: h264 encoder library
+;* MMX/SSE2-optimized H.264 deblocking code
 ;*****************************************************************************
 ;* Copyright (C) 2005-2008 x264 project
 ;*

libavcodec/x86/h264_idct_sse2.asm

 ;*****************************************************************************
-;* dct-a.asm: h264 encoder library
+;* SSE2-optimized H.264 iDCT
 ;*****************************************************************************
 ;* Copyright (C) 2003-2008 x264 project
 ;*