drm/i915: Initialise outparam for error return from wait_for_register

Just in case the caller passes in 0 for both slow&fast timeouts, make
sure we initialise the stack value returned. Add an assert so that we
don't make the mistake of passing 0 timeouts for the wait.

drivers/gpu/drm/i915/intel_uncore.c:2011 __intel_wait_for_register_fw() error: uninitialized symbol 'reg_value'.

References: 3f649ab ("treewide: Remove uninitialized_var() usage")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200916105022.28316-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_uncore.c

@@ -1993,13 +1993,14 @@ int __intel_wait_for_register_fw(struct intel_uncore *uncore,
 				 unsigned int slow_timeout_ms,
 				 u32 *out_value)
 {
-	u32 reg_value;
+	u32 reg_value = 0;
 #define done (((reg_value = intel_uncore_read_fw(uncore, reg)) & mask) == value)
 	int ret;
 
 	/* Catch any overuse of this function */
 	might_sleep_if(slow_timeout_ms);
 	GEM_BUG_ON(fast_timeout_us > 20000);
+	GEM_BUG_ON(!fast_timeout_us && !slow_timeout_ms);
 
 	ret = -ETIMEDOUT;
 	if (fast_timeout_us && fast_timeout_us <= 20000)