drm/amdgpu: make function uvd_v6_0_enc_get_destroy_msg static

The function uvd_v6_0_enc_get_destroy_msg is local to the source and
does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'uvd_v6_0_enc_get_destroy_msg' was not declared. Should it be
static?

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>