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: 98ab5f35

 ("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