drm/i915: rename i915_gem_alloc_object() to i915_gem_object_create()

Because having both i915_gem_object_alloc() and i915_gem_alloc_object()
(with different return conventions) is just too confusing!

(i915_gem_object_alloc() is the low-level memory allocator, and remains
unchanged, whereas i915_gem_alloc_object() is a constructor that ALSO
initialises the newly-allocated object.)

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461348872-4702-1-git-send-email-david.s.gordon@intel.com