drm/i915: Select X86_MSR for igt

igt/pm_rpm depends upon /dev/*/msr so automatically select it when
enabling i915 debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1468054147-9821-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>