configure: drop explicit DRI_DIRS assignment on some platforms/arches
configure: drop explicit DRI_DIRS assignment on some platforms/arches

Both x86_64|amd64 and *bsd, already set the full range of available
classic dri drivers. Drop the explicit assignment, and fall back to
the generic default.
Keep explicit list from plafroms/arches that do not handle the default
list.

Update help strings, to explicitly mention "classic" for applicable
DRI drivers.

Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

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