x86: make ioremap() UC by default

Yes! A mere 120 c_p_a() fixing and rewriting patches later,
we are now confident that we can enable UC by default for
ioremap(), on x86 too.

Every other architectures was doing this already. Doing so
makes Linux more robust against MTRR mixups (which might go
unnoticed if BIOS writers test other OSs only - where PAT
might override bad MTRRs defaults).

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>