gallium: Use enum pipe_shader_type in bind_sampler_states() (v2)

gallium: Use enum pipe_shader_type in bind_sampler_states() (v2)

v1 → v2:
 - Fixed indentation (noted by Brian Paul)
 - Removed second assert from nouveau's switch statements (suggested by
   Brian Paul)

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>