r600g: consolidate code for setting sampler views and fix bugs in the process

r600g: consolidate code for setting sampler views and fix bugs in the process

Issues fixed:

- set_vs_sampler_views for evergreen is now properly implemented.

- Added the missing inval_texture_cache call for evergreen.

- have_depth_texture was sometimes incorrectly set to false on evergreen even
  if there were depth textures in other shader stages. To fix this, set it
  to true once and never set it to false again. It's stupid, but it matches
  the r600 code. The proper fix is left to another patch.

- Optimizaton: The sampler views which aren't changed aren't updated.