dispatch: Make all API functions non-static.

dispatch: Make all API functions non-static.

Some of the functions that we store in the dispatch table are declared
as non-static in their .c files and are inserted into the dispatch
table directly by _mesa_create_exec_table().  Other functions are
declared as static, and are inserted into the dispatch table by a
dedicated function that lives in the same .c file
(e.g. _mesa_loopback_init_api_table() in api_loopback.c).

This patch makes all of these functions non-static, and creates
appropriate prototypes for them, so that in future patches we can
populate the entire dispatch table using a single code-generated
function.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>