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 <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: Nirmoy Das <[e-mail]>
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/selftests/test-drm_mm.c

···
 static bool assert_no_holes(const struct drm_mm *mm)
 {
 	struct drm_mm_node *hole;
-	u64 hole_start, hole_end;
+	u64 hole_start, __always_unused hole_end;
 	unsigned long count;
 
 	count = 0;