drm/i915/guc: make 'guc_hw_reg_state' static as it isn't exported

drm/i915/guc: make 'guc_hw_reg_state' static as it isn't exported

The guc_hw_reg_state array is not exported, so make it static.
Fixes the following sparse warning:
drivers/gpu/drm/i915/i915_gpu_error.c:692:3: warning: symbol 'guc_hw_reg_state' was not declared. Should it be static?

Fixes: ba391a102ec11 ("drm/i915/guc: Include the GuC registers in the error state")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patch.msgid.link/20260108201202.59250-2-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>