drm/i915: fix documentation build warnings

Just a straightforward bag of fixes for a clean htmldocs build.

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605095657.23601-2-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_vma.h

@@ -40,6 +40,8 @@
 enum i915_cache_level;
 
 /**
+ * DOC: Virtual Memory Address
+ *
  * A VMA represents a GEM BO that is bound into an address space. Therefore, a
  * VMA's presence cannot be guaranteed before binding, or after unbinding the
  * object into/from the address space.