meta: Refactor the table of glsl_sampler structures

meta: Refactor the table of glsl_sampler structures

This will allow the same table of shader-per-sampler-type to be used for
paths in meta other than just mipmap generation.  This is also the
reason the declarations of the structures was moved towards the top of
the file.

v2: Code formatting change suggested by Brian.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>