drm/i915/gem: Unify user object creation (v3)

Instead of hand-rolling the same three calls in each function, pull them
into an i915_gem_object_create_user helper.  Apart from re-ordering of
the placements array ENOMEM check, there should be no functional change.

v2 (Matthew Auld):
 - Add the call to i915_gem_flush_free_objects() from
   i915_gem_dumb_create() in a separate patch
 - Move i915_gem_object_alloc() below the simple error checks
v3 (Matthew Auld):
 - Add __ to i915_gem_object_create_user and kerneldoc which warns the
   caller that it's not validating anything.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723172142.3273510-5-jason@jlekstrand.net