drm/i915/display: Return early after MISSING_CASE for write_dp_sdp

Avoid using the uninitialised len along the impossible error path to
shut the compiler up:

drivers/gpu/drm/i915/display/intel_dp.c:4928 intel_write_dp_sdp() error: uninitialized symbol 'len'.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200325140754.12636-1-chris@chris-wilson.co.uk