oprofile/x86: rework error handler in nmi_setup()

This patch improves the error handler in nmi_setup(). Most parts of
the code are moved to allocate_msrs(). In case of an error
allocate_msrs() also frees already allocated memory. nmi_setup()
becomes easier and better extendable.

Signed-off-by: Robert Richter <robert.richter@amd.com>