drm/i915: add support for SBI ops

With Lynx Point, we need to use SBI to communicate with the display clock
control. This commit adds helper functions to access the registers via
SBI.

v2: de-inline the function and address changes in bits names

v3: protect operations with dpio_lock, increase timeout to 100 for
paranoia sake.

v4: decrease paranoia a bit, as noticed by Chris Wilson

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>