x86: mtrr fix debug boot parameter

while looking at:

  http://bugzilla.kernel.org/show_bug.cgi?id=11541

I realized that the mtrr.show param cannot work, because
the code is processed much too early.

This patch:
 - Declares mtrr.show as early_param
 - Stays consistent with the previous param (which I doubt
   that it ever worked), so mtrr.show=1 would still work
 - Declares mtrr_show as initdata

Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>