drm/i915: Extract CSB status read

drm/i915: Extract CSB status read

This is a useful thing to have around as a function because the mechanism may
change in the future.

There is a net increase in LOC here, and it will continue to be the case on GEN8
and GEN9 - but future GENs may have an alternate mechanism for doing this.

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452018609-10142-4-git-send-email-benjamin.widawsky@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>