drm/i915: Tidy up signed/unsigned comparison

drivers/gpu/drm/i915/i915_irq.c|3096 col 29| error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171130125253.21405-1-chris@chris-wilson.co.uk
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>