drm/i915/guc: Use correct lock for CT event handler

CT event handler is called under the gt->irq_lock from the interrupt
handling paths so make it the same from the init path. I don't think this
mismatch caused any functional issue but we need to wean the code of the
global i915->irq_lock.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120095636.1987395-2-tvrtko.ursulin@linux.intel.com