drm/amdgpu: Add parameter documentation for amdgpu_sync_fence

The 'flags' parameter, which specifies memory allocation behavior while
creating a sync entry,

Fixes the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c:162: warning: Function parameter or struct member 'flags' not described in 'amdgpu_sync_fence'

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>