softpipe: consolidate sampler-related arrays

softpipe: consolidate sampler-related arrays

Combine separate arrays for vertex/fragment/geometry samplers, etc into
one array indexed by PIPE_SHADER_x.

This allows us to collapse separate code for vertex/fragment/geometry
state into loops over the shader stage.  More to come.

Reviewed-by: José Fonseca <jfonseca@vmware.com>