drm/i915/uc: Drop use of MISSING_CASE on trivial enums

drm/i915/uc: Drop use of MISSING_CASE on trivial enums

We can rely on compiler to notify us if we miss any case.
This approach may also reduce driver size (reported ~4K).

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170331102652.177664-1-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>