drm/i915/dsi: Replace check with a (missing) MIPI sequence name

Names of the MIPI sequence steps are sequential and defined, no
need to check for the gaps. However in seq_name the MIPI_SEQ_END
is missing. Add it there, and drop unneeded NULL check in
sequence_name().

Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231103201831.1037416-9-andriy.shevchenko@linux.intel.com