i965/fs: Use dispatch_width instead of reg_width in alloc_reg_sets
i965/fs: Use dispatch_width instead of reg_width in alloc_reg_sets

reg_width is kind of an outdated concept.

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler