amdgpu: Move union declaration to top of amdgpu_cs_ctx_override_priority

Avoids compiler warning:

../../amdgpu/amdgpu_cs.c: In function 'amdgpu_cs_ctx_override_priority':
../../amdgpu/amdgpu_cs.c:155:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
  union drm_amdgpu_sched args;
  ^~~~~

Reviewed-by: Christian König <christian.koenig@amd.com>