drm/i915: Move i915_gem_fini to i915_gem.c

drm/i915: Move i915_gem_fini to i915_gem.c

We should keep i915_gem_init/fini functions together for easier
tracking of their symmetry.

v2: rebased, pulled out from the series

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180604090032.20840-1-michal.wajdeczko@intel.com