drm/amdgpu: Refine uvd_v6/7_0_enc_get_destroy_msg

1. make uvd_v7_0_enc_get_destroy_msg static
2. drop a function variable that always true

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>