x86/mtrr: Rename main.c to mtrr.c and remove duplicate prefixes

Kbuild uses the first file as the name for KBUILD_MODNAME.
mtrr uses main.c as its first file, so rename that file to mtrr.c
and fixup the Makefile.

Remove the now duplicate "mtrr: " prefixes from the logging calls.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lkml.kernel.org/r/ae1fa81a0d1fad87571967b91ea90f70f486e853.1525964384.git.joe@perches.com