softpipe: Put mip_filter_func inside a struct

softpipe: Put mip_filter_func inside a struct

Putting this function pointer into a struct enables grouping of
several related functions in a single place. For now it is just a
single function, but the struct will be later extended with a
mip_level_func for returning relative mip level.

v2:
  - renamed sp_mip struct to sp_filter_funcs
  - renamed sp_filter_funcs instances from mip_foo to funcs_foo
  - splitted too long lines
  - sp_sampler now holds a pointer to sp_filter_funcs instead of an
    instance of it
  - some const fixes

Reviewed-by: Brian Paul <brianp@vmware.com>