drm/i915/tc: Delete bogus NULL check in intel_ddi_encoder_destroy()

The "digi_port" pointer can't be NULL and we have already dereferenced
it so checking for NULL is not necessary.  Delete the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211004103737.GC25015@kili