drm/amd/display/amdgpu_dm/amdgpu_dm_helpers: Use 'gnu_printf' format notation

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

 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c: In function ‘dm_dtn_log_append_v’:
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c:345:2: warning: function ‘dm_dtn_log_append_v’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c:375:3: warning: function ‘dm_dtn_log_append_v’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>