drm/i915/irq: drop unused but set variable tmp

Prepare for re-enabling -Wunused-but-set-variable.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6c529e8721d56b0148a3a84fb2d396d4485e09a2.1685119007.git.jani.nikula@intel.com