drm/amdgpu: fix warning on older gcc releases

gcc-4.8 warns about '{0}' being used an an initializer for nested structures:

drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c: In function ‘gfx_v8_0_ring_emit_ce_meta_init’:
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c:7263:2: warning: missing braces around initializer [-Wmissing-braces]
  } ce_payload = {0};
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c: In function ‘gfx_v8_0_ring_emit_de_meta_init’:
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c:7290:2: warning: missing braces around initializer [-Wmissing-braces]
  } de_payload = {0};

Using an empty {} initializer however has the same effect and works on all versions.

Fixes: acad2b2a ("drm/amdgpu:implement CE/DE meta-init routines")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>