gallium/util: comments, var renaming in u_inlines.h
gallium/util: comments, var renaming in u_inlines.h

The variable 'usage' was being used for two different things.
Sometimes for PIPE_USAGE_x and other times for PIPE_TRANSFER_x.
This renames usage to access when we're talking about PIPE_TRANSFER_x
flags.  Plus, add a bunch of comments to remind us what's going on.

Also, use unsigned for PIPE_TRANSFER_x bitmask to be consistent with
other places.  And add a missing const qualifier.

Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

dellis1972/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,djreep81/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,metora/MesaGLSLCompiler,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,metora/MesaGLSLCompiler,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,dellis1972/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer