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: 3f649ab7

 ("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