drm/i915: Add a default case in gen7 hwsp switch-case

Gen7 won't get any new engines, and we already added VCS2 there to just
silence gcc's not handled in switch warnings.

Use a default case instead, otherwise we will need to keep adding extra
cases if changes happen in the future.

v2: Since reaching the default case is impossible, use GEM_BUG_ON (Chris).

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170830180115.907-1-michel.thierry@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>