x86/mtrr: Get rid of mtrr_seq_show() forward declaration

... by moving the function up in the file.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20191108200815.24589-1-bp@alien8.de