Move the AV_PERM_* flags definition outside the AVFilterPicRef

definition.

This way it is easier to reference them in other structures, for
example in the pending AVFilterSamplesRef struct.

Patch by S.N. Hemanth Meenakshisundaram smeenaks AT ucsd DOT edu.

Originally committed as revision 24294 to svn://svn.ffmpeg.org/ffmpeg/trunk