drm/selftests/test-drm_mm: Mark 'hole_end' as always_unused

drm/selftests/test-drm_mm: Mark 'hole_end' as always_unused

In the macro drm_mm_for_each_hole() 'hole_end' is provided as a
container for 'hole_start + pos->hole_size', but is not utilised in
this use-case.  We cannot simply delete the variable, so here we tell
the compiler that we're intentionally discarding the read value.

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/selftests/test-drm_mm.c: In function ‘assert_no_holes’:
 drivers/gpu/drm/selftests/test-drm_mm.c:57:18: warning: variable ‘hole_end’ set but not used [-Wunused-but-set-variable]

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Nirmoy Das <nirmoy.aiemd@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-28-lee.jones@linaro.org