drm/amd/display: fix unused variable warning

After all users of the 'dm' warnings got hidden in an #ifdef,
the compiler started warning about it being unused:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5380:33: error: unused variable 'dm' [-Werror,-Wunused-variable]

Add another such #ifdef.

Fixes: 98ab5f3 ("drm/amd/display: Fix deadlock during gpu reset v3")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210125124849.102037-1-arnd@kernel.org

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5374,7 +5374,9 @@ static inline int dm_set_vblank(struct drm_crtc *crtc, bool enable)
 	struct amdgpu_crtc *acrtc = to_amdgpu_crtc(crtc);
 	struct amdgpu_device *adev = drm_to_adev(crtc->dev);
 	struct dm_crtc_state *acrtc_state = to_dm_crtc_state(crtc->state);
+#if defined(CONFIG_DRM_AMD_DC_DCN)
 	struct amdgpu_display_manager *dm = &adev->dm;
+#endif
 	int rc = 0;
 
 	if (enable) {