drm/i915: Fix vma allocator debug

Add a missing colon which I accidentally removed in the recent logging
changes.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: a10234fd ("drm/i915: Partial abandonment of legacy DRM logging macros")
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115101730.394880-1-tvrtko.ursulin@linux.intel.com