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

drivers/gpu/drm/i915/gt/intel_ring_submission.c

@@ -767,7 +767,7 @@ static int mi_set_context(struct i915_request *rq,
 	if (GRAPHICS_VER(i915) == 7) {
 		if (num_engines) {
 			struct intel_engine_cs *signaller;
-			i915_reg_t last_reg = {}; /* keep gcc quiet */
+			i915_reg_t last_reg = INVALID_MMIO_REG; /* keep gcc quiet */
 
 			*cs++ = MI_LOAD_REGISTER_IMM(num_engines);
 			for_each_engine(signaller, engine->gt, id) {