drm/i915: Kerneldoc for i915_gem_evict.c

Request by Ben Widawsky in his review of a patch touching this code.

v2: Clarify the disdinction between evicting vmas (to free up virtual
address space) and evicting objects (to free up actual system memory).
Suggested by Ben.

Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Acked-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>