drm/nouveau: make vga_switcheroo code depend on VGA_SWITCHEROO

Commit 8116188f ("nouveau/acpi: hook up to the MXM method for mux
switching.") broke the build on non-x86 architectures due to the new
dependency on MXM and MXM being an x86 platform driver.

It built previously since the vga switcheroo registration routines were
zereod out on !X86.  The code was built in but unused.

This patch makes all of the DSM code depend on CONFIG_VGA_SWITCHEROO,
allowing it to build on non-x86 and shrinking the module size as well.

[rdunlap@infradead.org: fix build eror when VGA_SWITCHEROO is not enabled]
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>