gallium: consolidate CSO sampler and sampler_view functions

gallium: consolidate CSO sampler and sampler_view functions

Merge the vertex/fragment versions of the cso_set/save/restore_samplers()
functions.  Now we pass the shader stage (PIPE_SHADER_x) to the function
to indicate vertex/fragment/geometry samplers.  For example:

cso_single_sampler(cso, PIPE_SHADER_FRAGMENT, unit, sampler);

This results in quite a bit of code reduction, fewer CSO functions and
support for geometry shaders.

Reviewed-by: Marek Olšák <maraeo@gmail.com>