x86/mce: Unify pr_* prefix

Move the pr_fmt prefix to internal.h and include it everywhere. This
way, all pr_* printed strings will be prepended with "mce: ".

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20181205200913.GR29510@zn.tnic