drm/i915: intel_display.c handle latency variable efficiently

By handling latency variable efficiently we also get rid of this warning :

  CC [M]  drivers/gpu/drm/i915/intel_display.o
drivers/gpu/drm/i915/intel_display.c: In function ‘igd_enable_cxsr’:
drivers/gpu/drm/i915/intel_display.c:1918: warning: ‘latency’ may be used uninitialized in this function

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>