drm/i915/gt: fix i915_reg_t initialization

The initialization is there only to silence the compiler, but use the
correct initializer for i915_reg_t.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135955.922791-1-jani.nikula@intel.com