drm/i915/ttm: fix i915_ttm_to_gem() kernel-doc

drivers/gpu/drm/i915/gem/i915_gem_ttm.h:50: warning: Function parameter or member 'bo' not described in 'i915_ttm_to_gem'

v2: Fix return value documentation while at it (Rodrigo)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230504092320.1787627-1-jani.nikula@intel.com