x86/mtrr: Get rid of __mtrr_enabled bool

There is no need for keeping __mtrr_enabled as it can easily be replaced
by testing mtrr_if to be not NULL.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221102074713.21493-11-jgross@suse.com


Signed-off-by: Borislav Petkov <bp@suse.de>