drm/i915: add LCPLL control registers

Those are used to control the display core clock.

v2: change the enable bit setting, spotted by Rodrigo Vivi.

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