x86: msr-on-cpu: remove unnecessary level of abstraction

Remove an unnecessary level of abstraction in the msr-on-cpu library.
Although this duplicates some code, the duplicated code is less than
the additional code, and this way should be faster.

Additionally, change the order of the functions to make the regular
structure of this file more obvious.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>