drm/i915/lvds: Remove dead code from failing case

Coverity points out that, if we end up in the 'failed' label, that's
precisely because we couldn't retrieve a fixed mode (ie fixed_mode is
NULL) and then "if (fixed_mode)" is always false.

Remove that dead code.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>