program: convert _mesa_init_gl_program() to take struct gl_program *

program: convert _mesa_init_gl_program() to take struct gl_program *

Rather than accepting a void pointer, only to down and up cast around
it, convert the function to take the base (struct gl_program) pointer.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>