softpipe: Add functions for computing relative mipmap level

softpipe: Add functions for computing relative mipmap level

These functions will be used by textureQueryLod.

v2:

  - renamed mip_level_* funcs to mip_rel_level_* to indicate that
    these functions return mip level relative to base level and
    documented them
  - renamed a level member in sp_filter_funcs struct to relative_level
  - changed mip_rel_level_none and mip_rel_level_nearest to return mip
    level relative to base level, mip_rel_level_linear already did
    that
  - documented clamp_lod function

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