x86: mtrr: Restructure so command execution is in one place

At present do_mtrr() does the 'list' subcommand at the top and the rest
below. Update it to do them all in the same place so we can (in a later
patch) add parsing of the CPU number for all subcommands.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>