st/mesa/i965: simplify gl_program references and stop leaking

st/mesa/i965: simplify gl_program references and stop leaking

In i965 we were calling _mesa_reference_program() after creating
gl_program and then later calling it again with NULL as a param
to get the refcount back down to 1. This changes things to not
use _mesa_reference_program() at all and just have gl_linked_shader
take ownership of gl_program since refcount starts at 1.

The st and ir_to_mesa linkers were worse as they were both getting
in a state were the refcount would never get to 0 and we would leak
the program.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>