perf disassemble: Simplify logic for picking the filename to disassemble

perf disassemble: Simplify logic for picking the filename to disassemble

Lots of changes to support kcore, compressed modules, build-id files
left us with some spaguetti code, simplify it a bit, more to come.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-h70p7x451li3f2fhs44vzmm8@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>