drm/i915: Polish intel_tv_mode_valid()

Drop the tv_mode NULL check since intel_tv_mode_find() never
actually returns NULL, and flip the condition around so that
the MODE_OK case is at the end, which is customary to all
the other .mode_valid() implementations.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191001154629.11063-2-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>