drm/i915/selftests: Disable shrinker across mmap-exhaustion

For mmap-exhaustion, we deliberately put the system under a large amount
of pressure to ensure that we are able to reap mmap-offsets from dead
objects. If background activity does that reaping for us, that defeats
the purpose of the test and in some cases will fail our sanity checks
(because of the fake activity we use to prevent the idle worker).

Fixes: 932cac10

 ("drm/i915/selftests: Prevent background reaping of acti
ve objects")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181011103748.18387-1-chris@chris-wilson.co.uk
(cherry picked from commit 0b4bf7ca

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>