drm/i915: move more generic utils to i915_utils.h

Reduce clutter from i915_drv.h and intel_drv.h.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8c197872384fc35442b738c21ba0da9336e02a85.1556809195.git.jani.nikula@intel.com