mesa: separate exec allocation from initialization

mesa: separate exec allocation from initialization

In glapi/gl_genexec.py:
* Remove _mesa_alloc_dispatch_table call

In glapi/gl_genexec.py and api_exec.h:
* Rename _mesa_create_exec_table to _mesa_initialize_exec_table

In context.c:
* Call _mesa_alloc_dispatch_table instead of _mesa_create_exec_table
* Call _mesa_initialize_exec_table (this is temporary)

Once all drivers have been modified to call
_mesa_initialize_exec_table, then the call to
_mesa_initialize_context can be removed from context.c.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>