i965: Move and rename "wm sampler" fields to just "sampler".

i965: Move and rename "wm sampler" fields to just "sampler".

brw_wm_samplers actually enables any active samplers regardless of what
pipeline stage is using them, so it doesn't make much sense for it to be
WM-specific.  So, rename it to "brw_samplers."

To properly generalize it, move sampler_count and sampler_offset from
brw_context::wm to a new brw_context::sampler that can be shared without
looking strange.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>