x86/mtrr: Remove the mtrr_bp_init() stub

Add an IS_ENABLED() check in setup_arch() and call pat_disable()
directly if MTRRs are not supported. This allows to remove the
<asm/memtype.h> include in <asm/mtrr.h>, which pull in lowlevel x86
headers that should not be included for UML builds and will cause build
warnings with a following patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211215165612.554426-2-hch@lst.de