drm/i915: make some error capture functions static

Not needed outside of i915_gpu_error.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231031124502.1772160-1-jani.nikula@intel.com