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: NRodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NEugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>