drm/i915: avoid rebuilding i915_gpu_error.o on version string updates

There is no need to rebuild i915_gpu_error.o when the version string
changes as the version is available in init_utsname()->release.

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121095423.20760-1-hans.ml.holmberg@owltronix.com