drm/i915/display: Add default case to mipi_exec_send_packet

drm/i915/display: Add default case to mipi_exec_send_packet

Add a default case to the switch case statement in mipi_exec_send_packet
to prevent attempts to read an uninitialized ret value.  It's unlikely
the default case will ever occur during regular exeuction, but if more
MIPI DSI Processor-to-Peripheral transaction types are ever added, then
having this in place will be a useful safety guard.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Fixes: 23db1577ce2d ("drm/i915/dsi: log send packet sequence errors")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20251104164150.16795-2-jonathan.cavitt@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>