powerpc/ftrace: Refactor ftrace_{en/dis}able_ftrace_graph_caller

ftrace_enable_ftrace_graph_caller() and
ftrace_disable_ftrace_graph_caller() have common code.

They will have even more common code after following patch.

Refactor into a single ftrace_modify_ftrace_graph_caller() function.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f37785a531f1a8f201e1b3da45997a5c77e9d820.1640017960.git.christophe.leroy@csgroup.eu