draw: rename bind_sampler_states variables

Put 'fragment' in the names.  In preparation for upcoming function
renaming.
r300g: rename r300_bind_sampler_states to r300_bind_fragment_sampler_states
gallium: add pipe_context::bind_sampler_states()

The bind_vertex/geometry/fragment/compute_sampler_states() functions
will be replaced by a single functions.
cso: use pipe_context::bind_sampler_states() if non-null
draw: use pipe_context::bind_sampler_states() if non-null
util: use pipe_context::bind_sampler_states() if non-null
vl: use pipe_context::bind_sampler_states() if non-null
clover: use pipe_context::bind_sampler_states() if non-null
galahad: implement pipe_context::bind_sampler_states()
i915g: implement pipe_context::bind_sampler_states()
identity: implement pipe_context::bind_sampler_states()
ilo: implement pipe_context::bind_sampler_states()
llvmpipe: implement pipe_context::bind_sampler_states()
noop: implement pipe_context::bind_sampler_states()
rbug: implement pipe_context::bind_sampler_states()
trace: implement pipe_context::bind_sampler_states()
svga: implement pipe_context::bind_sampler_states()
radeon: implement pipe_context::bind_sampler_states()
softpipe: implement pipe_context::bind_sampler_states()
nouveau: implement pipe_context::bind_sampler_states()