drm/i915: Extract reserving space in the GTT to a helper

Extract drm_mm_reserve_node + calling i915_gem_evict_for_node into its
own routine so that it can be shared rather than duplicated.

v2: Kerneldoc

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: igvt-g-dev@lists.01.org
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170111112312.31493-2-chris@chris-wilson.co.uk