drm/i915: Fix up kerneldoc parameters for i915_gem_gtt_*()

drm/i915: Fix up kerneldoc parameters for i915_gem_gtt_*()

Parameter: good.
Parameter - bad.

One day I'll learn the syntax.

Fixes: 625d988acc28 ("drm/i915: Extract reserving space in the GTT to a helper")
Fixes: e007b19d7ba7 ("drm/i915: Use the MRU stack search after evicting")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170112164559.27232-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>