Hide gpu error specifics in i915_gpu_error.c. This is also cleaner wrt
conditional compilation, as i915_gpu_error.c is only built with
DRM_I915_CAPTURE_ERROR=y.

With this, we can also make i915_first_error_state() static.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231031124502.1772160-3-jani.nikula@intel.com