drm/amd/display: small cleanup in destruct()

Static analysis tools get annoyed that we don't indent this if
statement.  Actually, the if statement isn't required because kfree()
can handle NULL pointers just fine.  The DCE110STRENC_FROM_STRENC()
macro is a wrapper around container_of() but it's basically a no-op or a
cast.  Anyway, it's not really appropriate here so it should be removed
as well.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>